USB: phy-msm-qusb: Set ref_clk_src to NULL if clk_get fails

Set ref_clk_src to NULL if clk_get() API fails to get handle
in probe(), else it results in invalid pointer derefernce.

Change-Id: I1ba5f64bc2a691923cbac1c7b4e9288b6071ff57
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>