S3C24XX: ensure we only configure valid GPIOs

If we specify an GPIO which cannot be used for the purpose, then assume
that the GPIO is not to be used and do not try and configure it.  This can
be the case where the SPI bus is TX only.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>