wifi: mt76: split get_of_eeprom in subfunction

In preparation for NVMEM support, split get_of_eeprom() in subfunction
to tidy the code and facilitate the addition of alternative method to
get eeprom data. No behaviour change intended.

While at it also drop OF ifdef checks as OF have stubs and calling
of_get_property would result in the same error returned.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>