Staging: RTL8192SU depends on USB

USB device driver needs to depend on USB to prevent build errors:

ERROR: "usb_kill_urb" [drivers/staging/rtl8192su/r8192s_usb.ko] undefined!
ERROR: "usb_deregister" [drivers/staging/rtl8192su/r8192s_usb.ko] undefined!
ERROR: "usb_control_msg" [drivers/staging/rtl8192su/r8192s_usb.ko] undefined!
ERROR: "usb_submit_urb" [drivers/staging/rtl8192su/r8192s_usb.ko] undefined!
ERROR: "usb_register_driver" [drivers/staging/rtl8192su/r8192s_usb.ko] undefined!
ERROR: "usb_free_urb" [drivers/staging/rtl8192su/r8192s_usb.ko] undefined!
ERROR: "usb_alloc_urb" [drivers/staging/rtl8192su/r8192s_usb.ko] undefined!
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/rtl8192su/Kconfig

 config RTL8192SU
 	tristate "RealTek RTL8192SU Wireless LAN NIC driver"
-	depends on PCI && WLAN
+	depends on PCI && WLAN && USB
 	depends on WIRELESS_EXT
 	default N
 	---help---