usb: dwc3-msm: Switch master clock rate to XO when disabling

After the USB driver disables its clocks, such as during cable
disconnect, when the system enters XO shutdown, the USB30
master clock may experience clock glitches when its PLL is
forcibly shut off. To avoid this, switch the USB30 master
clock rate to XO (19.2 MHz) just before disabling it.

Change-Id: I1951a7877ce7b4c8cb86577f5ba921fd17a6ad63
Signed-off-by: Jack Pham <jackp@codeaurora.org>