iwlegacy: set tx power after rxon_assoc

If settings of tx power was deferred during scan or changing channel we
have to setup them during commit rxon. Fix problem on 3945 (4965 already
has this fix).

Optimize code to apply tx settings only when tx power was actually
changed.

Cc: stable@kernel.org # 2.6.39+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>