iwlwifi: pcie: prevent using unmapped memory in fw monitor

In the case of a DMA mapping error on the last iteration of
the loop of the allocation of memory of the FW monitor we
indeed free the pages, but don't NULL out the page variable
thus allowing for the possibility of setting the FW monitor
variables with invalid data to use.

Fixes: c2d20201

 ("iwlwifi: pcie: add firmware monitor capabilities")
Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>