staging: rtl8188eu: Add 'rtl8188eufw.bin' to MODULE_FIRMWARE list

This is the file loaded by the code anyway, but now you can use
'modinfo' to determine the needed firmware file for this module.

Spotted when packaging firmware files for the fli4l Linux router
distribution, where a script uses the information from 'modinfo' to
collect all needed firmware files to package.

Cc: Christoph Schulz <mail@kristov.de>
Signed-off-by: Alexander Dahl <post@lespocky.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>