mac80211: A couple of fixes to dynamic power save.

a) hw_config() should not be called from siwpower() for the drivers which do not support
   dynamic powersave.
b) IEEE80211_HW_NO_STACK_DYNAMIC_PS needs to be verified in set_associated() also before
   enabling the power save timers.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>