staging: comedi: remove 'bc.subdevice < 0' check in do_bufconfig_ioctl()

Type of variable 'bc' is struct comedi_bufconfig and 'subdevice' is
defined as unsigned, so there is no need to check if it is less then
zero.

Signed-off-by: Güngör Erseymen <gelurine@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>