Staging: comedi: fix signal handling in read and write

After sleeping on a wait queue, signal_pending(current) should be
checked (not before sleeping).

Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>