rtlwifi: usb parts should depend on CONFIG_USB

ERROR: "usb_unanchor_urb" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_control_msg" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_submit_urb" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_get_dev" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_kill_anchored_urbs" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_put_dev" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_free_urb" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_anchor_urb" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
ERROR: "usb_alloc_urb" [drivers/net/wireless/rtlwifi/rtlwifi.ko] undefined!
make[2]: *** [__modpost] Error 1
make[1]: *** [modules] Error 2
make: *** [sub-make] Error 2

The USB-part of rtlwifi should depend on CONFIG_USB.  This also
corrects the existing check for CONFIG_PCI to build pci.o.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>