iwlwifi: mvm: unmap the paging memory before freeing it

This led to a DMA splat.

Fixes: a6c4fb44

 ("iwlwifi: mvm: Add FW paging mechanism for the UMAC on PCI")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>