staging: comedi: only set dev->n_subdevices when kcalloc succeedes

It's possible for the kcalloc in comedi_alloc_subdevices to fail.
Only set the dev->n_subdevices variable if the allocation is
successful.

Since the core sets dev->n_subdevices, remove all the places in the
drivers where this variable was getting set.

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>