viafb: call viafb_get_clk_value only in viafb_set_vclock

As no caller is interested in the result call viafb_get_clk_value
directly from viafb_set_vclock to encapsulate the hardware dependend
stuff there.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>