staging: comedi: pcmmio: remove unneeded checks on detach

As pointed out by Dan carpenter for the similar pcmuio driver, there is
no need to check the pointer passed to `kfree()`, so remove that check
from `pcmmio_detach()`.

Also, check the `devpriv` (`dev->private`) pointer once, outside the
`for` loop.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>