iwlwifi: mvm: fix failure path when power_update fails in add_interface

When iwl_mvm_power_update_mac() is called, we have already added the
mac context, so if this call fails we should remove the mac.

CC: <stable@vger.kernel.org> [3.15+]
Fixes: commit e5e7aa8e ('iwlwifi: mvm: refactor power code')
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>