mac80211: rt2x00: improve eeprom_file property handling

Call the rt2x00lib_get_eeprom_file_name only once and from the function
where the EEPROM filename is required.

Error only out if an EEPROM file is mandatory. Use the
REQUIRE_EEPROM_FILE bit to determine if it is mandatory.

Do not set the REQUIRE_EEPROM_FILE bit while requesting an EEPROM file.
It should be (and is) set before requesting an EEPROM file.

Do not redirect users to upstream while using a function of a custom
patch.

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