Staging: rspiusb: Check usb_buffer_map_sg() retval

usb_buffer_map_sg() may return -1, check this directly.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Reviewed-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>