iwlwifi: mvm: Fix paging memory leak

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>