rt2x00: rt2800lib: introduce rt2800_eeprom_word enum

The patch converts the EEPROM_* word address defines
into new enum values. The new enum type will be used
by new functions which will be introduced in subsequent
changes.

The patch contains no functional changes.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>