Backport patch merged upstream that optimize the QDMA rx queue descriptor
setup by configuring the CPU IDX only when needed.
(cherry picked from commit 70ad03e0ac2d7c6d8597e13d74bbc057bb6fc17c)
Link: https://github.com/openwrt/openwrt/pull/23046
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>