comedi_fops: check s->async for read(), write() and poll() handlers

comedi_fops: check s->async for read(), write() and poll() handlers

Some comedi drivers set dev->read_subdev or dev->write_subdev to a
subdevice that does not set asynchronous commands.  This can cause a
NULL pointer dereference in comedi_read(), comedi_write(), or
comedi_poll() when they try to dereference s->async.

Test to make sure s->async is non-NULL.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>