iwlwifi: mvm: dump Rx FIFO when the firmware asserts

The Rx FIFO includes valuable data - dump it when the FW
asserts. Also - free the SRAM and Rx FIFO when we create
the file, and don't collect new SRAM / Rx FIFO if the
previous file hasn't been collected through debugfs yet.

Also - add a comment to saying that the ASSERT output should
not be modified since we have automatic scripts that monitor
this output.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>