iwlwifi: mvm: Fix paging memory leak

commit 905e36ae172c83a30894a3adefab7d4f850fcf54 upstream.

If the opmode is stopped and started again we did not free
the paging buffers. Fix that.
In addition when freeing the firmware's paging download
buffer, set the pointer to NULL.

Signed-off-by: Matti Gottlieb <matti.gottlieb@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>