staging: comedi: remove 'bi.subdevice < 0' check in do_bufinfo_ioctl()

Type of variable 'bi' is struct comedi_bufinfo 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>