wifi: mt76: Move RCU section in mt7996_mcu_add_rate_ctrl_fixed()

wifi: mt76: Move RCU section in mt7996_mcu_add_rate_ctrl_fixed()

Since mt7996_mcu_set_fixed_field() can't be executed in a RCU critical
section, move RCU section in mt7996_mcu_add_rate_ctrl_fixed() and run
mt7996_mcu_set_fixed_field() 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-v1-3-d46d15f9203c@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>