s3c2410fb: don't treat NULL clk as an error

clk_get() returns a struct clk cookie to the driver and some platforms
may return NULL if they only support a single clock.  clk_get() has only
failed if it returns a ERR_PTR() encoded pointer.

Cc: Vincent Sanders <support@simtec.co.uk>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>