usb: dwc3-msm: Allow xo_clk to be optional

Some instances of USB controller get XO directly from the
pad and hence don't need a GCC clock to enable it. Allow
xo_clk to be optional if not specified in DT.

Speaking of optional clocks, remove the NULL checks and allow
clk_* APIs to get called unconditionally, since they already
handle NULL pointers. Finally remove the clk_put() call for
xo_clk since it is already devm managed.

Change-Id: I1864219b89c7a636989f44fa7d6640bf5f156453
Signed-off-by: Jack Pham <jackp@codeaurora.org>