iwlwifi: mvm: fix memory leak in paging

Currently paging download buffer is freed during the
the unloading of the opmode which happens when the driver
is unloaded.

This causes a memory leak since the paging download
buffer is allocated every time we enable the
interface, so the download buffer can be allocated many
times, but only be freed once.

Free paging download buffer during disabling of the
interface.

CC: stable@vger.kernel.org [4.3+]
Signed-off-by: Matti Gottlieb <matti.gottlieb@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>