Fix u8 cast reading max_nss from MT_TOP_STRAP_STA register in
mt7615_eeprom_parse_hw_cap routine

Fixes: acf5457f ("mt76: mt7615: read {tx,rx} mask from eeprom")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>