ath5k: allow setting txpower to 0

As a holdover from earlier code when we used to set
the power limit to '0' after a reset to configure the
default transmit power, ath5k interprets txpower=0 as
12.5 dBm.  Fix that by just passing 0 through.

This fixes http://bugzilla.kernel.org/show_bug.cgi?id=14567



Cc: stable@kernel.org
Reported-by: Daniel Folkers <daniel.folkers@task24.nl>
Tested-by: Daniel Folkers <daniel.folkers@task24.nl>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>