staging: slicoss: return early before calling request_firmware with empty firmware filename

when the device id doesn't match in slic_card_download_gbrcv the filename is "",
i.e an empty name, and we try calling request_firmware with that name,
actually we can just fail out at default case before even calling request_firmware

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>