staging: comedi: don't return minor from comedi_alloc_subdevice_minor()

`comedi_alloc_subdevice_minor()` currently returns the allocated minor
device number on success.  This is not really of any interest to the
caller (in fact the return value is not even checked), so just return 0
on success.  If the caller really needs to know the allocated minor
device number it can look in `s->minor`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>