mt76: mt7615: fix EEPROM buffer size

Avoid adding MT7615_EEPROM_SIZE twice.
Rename MT7615_EEPROM_EXTRA_DATA to MT7615_EEPROM_FULL_SIZE, since it already
includes MT7615_EEPROM_SIZE

Fixes: ad380ad1 ("mt76: mt7615: add support for applying DC offset calibration from EEPROM")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200622150716.49622-1-nbd@nbd.name