wifi: mt76: mt7996: fix integer handling issue of mt7996_rf_regval_set()

This code is supposed to set a u32 value, but casting will not work on
big endian systems.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527816 ("Integer handling issues")
Fixes: 98686cd2

 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>