wifi: mt76: mt7921: move mt7921_rx_check and mt7921_queue_rx_skb in mac.c

Since both mt7921_rx_check and mt7921_queue_rx_skb routines are used by
all chipsets (mmio, usb and sdio), move them in mac.c and remove
duplicated code.

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