video: s3c-fb: fix possible division by zero in s3c_fb_calc_pixclk

Divider value can be zero and it makes division by zero
from debug message in s3c_fb_calc_pixclk; therefore, it
should be fixed.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>