dfu: Fix dfu_config_interfaces() for single interface DFU syntax

As stated in DFU documentation [1], the device interface part might be
missing in dfu_alt_info:

    dfu_alt_info
        The DFU setting for the USB download gadget with a semicolon
        separated string of information on each alternate:
            dfu_alt_info="<alt1>;<alt2>;....;<altN>"
        When several devices are used, the format is:
            - <interface> <dev>'='alternate list (';' separated)

So in first case dfu_alt_info might look like something like this:

    dfu_alt_info="mmc 0=rawemmc raw 0 0x747c000 mmcpart 1;"

And in second case (when the interface is missing):

    dfu_alt_info="rawemmc raw 0 0x747c000 mmcpart 1;"

When the interface is not specified the 'dfu' command crashes when
called using 'dfu 0' or 'dfu list' syntax:

    => dfu list
    "Synchronous Abort" handler, esr 0x96000006, far 0x0

That's happening due to incorrect string handling in
dfu_config_interfaces(). In case when the interface is not specified in
dfu_alt_info it triggers this corner case:

    d = strsep(&s, "=");  // now d contains s, and s is NULL
    if (!d)
        break;
    a = strsep(&s, "&");  // s is already NULL, so a is NULL too
    if (!a)               // corner case
        a = s;            // a is NULL now

which causes NULL pointer dereference later in this call, due to 'a'
being NULL:

    part = skip_spaces(part);

That's because as per strsep() behavior, when delimiter ("&") is not
found, the token (a) becomes the entire string (s), and string (s)
becomes NULL. To fix that issue assign "a = d" instead of "a = s",
because at that point variable d actually contains previous s, which
should be used in this case.

[1] doc/usage/dfu.rst

Fixes: commit febabe3ed4 ("dfu: allow to manage DFU on several devices")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Link: https://lore.kernel.org/r/20250709042342.13544-1-semen.protsenko@linaro.org
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>

drivers/dfu/dfu.c

@@ -147,7 +147,7 @@ int dfu_config_interfaces(char *env)
 			break;
 		a = strsep(&s, "&");
 		if (!a)
-			a = s;
+			a = d;
 		do {
 			part = strsep(&a, ";");
 			part = skip_spaces(part);