staging: comedi: sanity check num_subdevices parameter in comedi_alloc_subdevices

It's possible for a couple of the comedi drivers to incorrectly call
comedi_alloc_subdevices with num_subdevices = 0. Add a sanity check
before doing the kcalloc.

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