viafb: always return the best possible clock

Before this patch only clocks that perfectly match were used and if
none existed this was not handled properly. This patch changes this
to always use the closest clock supported. This should behave like
before for clocks that have a perfect match but be much saner for
clocks which are slightly off.

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