rt2x00: rt2800lib: introduce rt2800_get_txmixer_gain_{24,5}g helpers

Move the TX mixer gain reading code into separate
helper functions in preparation for RT3593 support.

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>