clk: qcom: Fix race condition between enable and set_rate

The parent field is not modified when switching to the safe_parent, so
refering to this in the enable op can get a stale value. Use the src_sel
field instead, which is modified with the appropriate lock held when
switching to the safe parent.

CRs-fixed: 618986
Change-Id: Ic6b5a19137ad723e72ae68e9266ca70e31a51c49
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>