fbdev: exynos: fix IS_ERR_VALUE usage

IS_ERR_VALUE macro should be used only with unsigned long type.
For signed types comparison 'ret < 0' should be used.

The patch follows conclusion from discussion on LKML [1][2].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2120927
[2]: http://permalink.gmane.org/gmane.linux.kernel/2150581



Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>