usb: otg: msm: Convert to clk_prepare/unprepare

Add calls to clk_prepare and unprepare so that MSM can migrate to
the common clock framework.

Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>