wifi: mt76: fix uninitialized pointer in mt7921_mac_fill_rx

Initialize msta pointer to NULL in mt7921_mac_fill_rx() in order to not
dereference a uninitialized pointer.

Fixes: 0880d408 ("mt76: connac: move mt76_connac2_reverse_frag0_hdr_trans in mt76-connac module")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>