rt2x00: rt2800lib: add rt2800_txpower_to_dev helper

Introduce a new helper function for converting
the default TX power values from EEPROM into
mac80211 values.

The change improves the readability and it makes
it easier to add support for other chipsets.

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>