staging: comedi: make determination of read or write subdevice safer

`comedi_read_subdevice()` and `comedi_write_subdevice()` respectively
determine the read and write subdevice to use for a comedi device,
depending on a minor device number passed in.  The comedi device has a
main "board" minor device number and may also have dynamically assigned,
subdevice-specific minor device numbers, in a range of numbers shared by
all comedi devices.  If the minor device number is within the range of
subdevice-specific minor device numbers, both functions call
`comedi_subdevice_from_minor()` to determine what subdevice is
associated with the minor device number (if any) and then check the
subdevice belongs to the comedi device.  Since the subdevice might
belong to a different comedi device, the check is not protected against
the subdevice being freed.  Perform the check in
`comedi_subdevice_from_minor()` instead, where it is protected against
the subdevice being freed.  Make it return `NULL` if the subdevice does
not belong to the device.

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