staging: comedi: comedi_fops: use kzalloc in comedi_alloc_subdevice_minor()

The hardware_device member of comedi_file_info is not set in this
function. Use kzalloc to make sure this pointer does not contain
invalid data.

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>