iwlwifi: move firmware restart debugfs hook to op_mode

iwlwifi: move firmware restart debugfs hook to op_mode

This allows to test fw restart flow. The hook in transport
layer doesn't really make the fw assert. Moving this hook
to the op_mode allows to use the fw API to actually send a
host command that will make the fw assert.

Change the restart_fw module parameter to be a boolean on
the way.

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