mt76x2: fix tx_alc_enabled check

Fix mt76x2_temp_tx_alc_enabled routine since in order to enable tx_alc
temperature compensation it necessary to take into account BIT(15) of
MT_EE_TX_POWER_EXT_PA_5G eeprom info

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>