iwlwifi: mvm: catch underflow error earlier

My static checker complains that we don't check for underflows in
iwl_dbgfs_fw_dbg_conf_write().  This is harmless because we have a
sanity check in iwl_mvm_start_fw_dbg_conf(), but we may as well make
this unsigned and silence the underflow warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>