rt2x00: Let rt2800lib check CONFIG_RT2X00_LIB_USB instead of CONFIG_RT2800USB

rt2800lib currently checks whether RT2800USB is enabled in the configuration.
Strictly speaking this is not necessary, it only needs to know whether the
generic rt2x00usb library functions are available. Therefore check for
RT2X00_LIB_USB instead.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>