staging: comedi: comedi_fops: cleanup comedi_poll()

Consolidate the local variables 'read_subdev' and 'write_subdev' into a
single local variable 's'.

Use comedi_dev_from_minor() to simplify the return -ENODEV tests.

Use a goto in the !dev->attached test so that the mutex_unlock() call
is in a common place.

Cleanup the formating of the || in the read and write subdevice poll
code.

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>