mac80211: rt2x00: remove eeprom filename dependency from mtd-eeprom

The ralink,mtd-eeprom device tree property doesn't require the
ralink,eeprom property to work.

Rework the error handling and user notification as well. Do not log an
error if the mtd-eeprom parameter isn't used. It could be intentional
and should not scare the user.

Check if the number of bytes read from the mtd devices matches the
requested number of bytes.

In case of an mtd read error, give a hint to the user which partition
was tried to read from.

In case everything is fine, notify the user as well.

Signed-off-by: Mathias Kresin <dev@kresin.me>