USB: PHY: msm: don't fail probe if snoc is not present

fermimum does not have snoc clock, for such targets 0 clk rate
for snoc is passed from dt. If snoc rate is read 0 from dt, do
not perform any clock operation for snoc.

Change-Id: Ifff0cce80039ebc0d900649d3ed90cf84e6b2a19
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>