wifi: mt76: Move RCU section in mt7996_mcu_set_fixed_field()

wifi: mt76: Move RCU section in mt7996_mcu_set_fixed_field()

Since mt76_mcu_skb_send_msg() routine can't be executed in atomic context,
move RCU section in mt7996_mcu_set_fixed_field() and execute
mt76_mcu_skb_send_msg() in non-atomic context. This is a preliminary
patch to fix a 'sleep while atomic' issue in mt7996_mac_sta_rc_work().

Fixes: 0762bdd30279 ("wifi: mt76: mt7996: rework mt7996_mac_sta_rc_work to support MLO")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250605-mt7996-sleep-while-atomic-v1-2-d46d15f9203c@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>