staging: rtl8192u: declare MODULE_FIRMWARE

declaring MODULE_FIRMWARE has apparently forgotten while removing the embedded
firmware arrays in 0a8692b5 (rtl8192u_usb:
Remove built-in firmware images).

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Cc: stable <stable@kernel.org> [2.6.39+]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>