staging: comedi: propogate error code from comedi_alloc_subdevices

comedi_alloc_subdevices can fail with -EINVAL or -ENOMEM. When it
does fail make sure to pass the proper error code back.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbott@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>