libertas: Improvements on automatic tx power control via SIOCSIWTXPOW (fixups)

This patch addresses comments from Dan Williams about the patch
committed as "libertas: Improvements on automatic tx power control via
SIOCSIWTXPOW."

Signed-off-by: Anna Neal <anna@cozybit.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>