mt76: mt7921: introduce mt7921_mcu_set_beacon_filter utility routine

Introduce mt7921_mcu_set_beacon_filter utility routine in order to
remove duplicated code for hw beacon filtering.
Move mt7921_pm_interface_iter in debugfs since it is just used there.
Make the following routine static:
- mt7921_pm_interface_iter
- mt7921_mcu_uni_bss_bcnft
- mt7921_mcu_set_bss_pm

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>