ath5k: Track current TX power separately from max TX power

Add a new variable to keep track of the currently configured tx power. Before
max_pwr was re-used for keeping the maximum allowed power as well as the
current configuration. Doing a min() on it allows you to lower the txpower, but
how would you be able to make it higher again?

This patch fixes that by adding a new variable ah_cur_pwr which is used instead
of txp_max_pwr to keep the current configuration. txp_max_pwr is used to check
if we are within the limits.

Another problem fixed by this patch is that it avoids setting a zero txpower
when things are initialized first and the current power is not yet set.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>