rt2x00: rt2800lib: remove TXPOWER_DELTA entry from extended EEPROM map

The TXPOWER_DELTA field of the regular EEPROM
stores the TX power compensation value for HT40.
The extended EEPROM has no such field, it stores
separate TX power values for HT20 and for HT40.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>