usb: gadget: ci13xxx: Do not clear USBCMD_SESS_VLD_CTRL on disconnect

Clearing the bit SESS_VLD_CTRL of the USBCMD register on cable
disconnect leads to leakage current in PHY if the USB regulators
are still ON.
Avoid this issue by not clearing the bit on USB cable disconnect.

Change-Id: I67ec52ba95f31d5c108256b248f08a2c8ab60c23
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>