ath5k: Put power_level where it belongs and rename it

Put power_level to ah_txpower struct with the rest tx power infos and
also rename it to txp_requested to make more sense.

v2 make sure we don't memset it to zero on reset

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>