mt76: mt76x02u: fix sparse warnings: should it be static?

Fix following sparse warnings in mt76x02_usb_core.c

drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c:29:6: warning:
symbol 'mt76x02u_tx_complete_skb' was not declared. Should it be static?
drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c:37:5: warning:
symbol 'mt76x02u_skb_dma_info' was not declared. Should it be static?
drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c:96:52: warning:
restricted __le16 degrades to integer
drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c:74:5: warning:
symbol 'mt76x02u_tx_prepare_skb' was not declared. Should it be static?
drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c:244:6: warning:
symbol 'mt76x02u_init_beacon_config' was not declared. Should it be
static?
drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c:262:6: warning:
symbol 'mt76x02u_exit_beacon_config' was not declared. Should it be
static?

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