mt76x0: rename mt76_* functions

mt76_* functions conflicts with mt7601u driver what prevents to build
those drivers in the kernel or use both drivers modules at once.

Patch fixes build errors like this:
ld: drivers/net/wireless/mediatek/mt76/mt76x0/mac.o:(.opd+0x30): multiple definition of `mt76_mac_tx_rate_val'; drivers/net/wireless/mediatek/mt7601u/mac.o:(.opd+0x30): first defined here

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 7b485902 ("mt76x0: core files")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>