iwlwifi: mvm: mark ret as maybe_unused in iwl_dbgfs_fw_restart_write()

iwlwifi: mvm: mark ret as maybe_unused in iwl_dbgfs_fw_restart_write()

The value returned from iwl_mvm_send_cmd_pdu() in this function is not
used and always returns an error (which is the whole point of the
call).  Tag the ret variable with __maybe_unused to avoid this compile
warning when W=1 is used:

/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/mvm/debugfs.c: In function ‘iwl_dbgfs_fw_restart_write’:
/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/mvm/debugfs.c:875:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
  int ret;
      ^

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>