rt2x00: rt2800lib: introduce rt2800_eeprom_word_index helper

Instead of assign the offset value to the
enum directly use a new helper function to
convert a rt2800_eeprom_word enum into an
index of the rt2x00_dev->eeprom array.

The patch does not change the existing
behaviour, but makes it possible to add
support for three-chain devices which are
using a different EEPROM layout.

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