OMAP2xxx clock: fix dss2_fck recalc to use clksel

dss2_fck is a clksel clock, and therefore its rate should be recalculated
with the clksel mechanism.  This was working in early 2009, but was one of
the casualties of the big OMAP clock merge between 2.6.29 and 2.6.30.

Signed-off-by: Paul Walmsley <paul@pwsan.com>