staging: comedi: don't override read/write subdevice if not supported

For comedi devices that support asynchronous commands on some of their
subdevices, the comedi core creates extra device files for each of those
subdevices of the form "/dev/comedi%i_subd%i".  These use the same
comedi device as the corresponding "/dev/comedi%i" but override the
default "read" and "write" subdevice for the device.  Currently it
overrides both the read and write subdevice, but it only makes sense to
override the "read" subdevice if the subdevice supports "read" commands,
and to override the "write" subdevice if the subdevice supports "write"
commands.

In `comedi_alloc_subdevice_minor()`, only set `info->read_subdevice`
non-NULL if the subdevice has the `SDF_CMD_READ` flag set, and only set
`info->write_subdevice` non-NULL if the subdevice has the
`SDF_CMD_WRITE` flag set.  (`comedi_read_subdevice(info)` will use the
device's default read subdevice if `info->read_subdevice` is NULL.
`comedi_write_subdevice(info)` will use the device's default write
subdevice if `info->write_subdevice` is NULL.

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