mt76: mt7915: fix endian bug in mt7915_rf_regval_set()

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

Fixes: 0a17329a

 ("mt76: mt7915: add debugfs knob for RF registers read/write")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>