staging: comedi: comedi_fops: cleanup comedi_{read, write}_subdevice()

Flip the info->device tests so than the return NULL occurs last.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>