rt2x00: rt2800lib: introduce rt2800_eeprom_read_from_array helper

Add a new helper function and use that for reading
single elements of various arrays in the EEPROM.

The patch does not change the current behaviour,
but it allows to use sequential values for the
rt2800_eeprom_word enums. The conversion will be
implemented in a subsequent change.

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>