iwmc3200wifi: Dont set the UMAC power limit when interface is down

When we're down, we shouldnt try to set the UMAC power limit. We just return 0
instead, and cfg80211 toggles the soft rfkill state.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>