iwlwifi: unmap memory before use

Handling responses to driver originated commands include passing the
original command buffer to the caller. At this time it is possible for a
callback to be invoked that is passed this command buffer and thus likely
to access it.

We need to make sure that the memory associated with that buffer is not DMA
mapped at the time.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>