staging: comedi: change type of num_subdevices parameter to comedi_alloc_subdevices

The n_subdevices variable of struct comedi_device is an int type.
Change the type of the comedi_alloc_subdevices 'num_subdevices' from
an unsigned int to an int to match it.

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>