msm: acpuclock-8960: Clean up init_clk_sources()

The function can be simplified. For correctness, it should also
take care to set the secondary mux to match the target frequency
before returning.  With current frequency tables this isn't a
problem, but could be if tgt_s at init was ever changed to be
from QSB.

Change-Id: Ie25a9e958acec7a9e4ac6d4820081bd446b05332
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>