wifi: mt76: limit support of precal loading for mt7915 to MTD only

wifi: mt76: limit support of precal loading for mt7915 to MTD only

Limit support for precal loading for mt7915 only to MTD. Passing data
from DT doesn't support offset and NVMEM require a different cell name
and doesn't support offset hence only MTD way is actually supported.

Rename mt76_get_of_eeprom_from_mtd to mt76_get_of_data_from_mtd as it is
now used for a more generic purpose and export it.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>