usb: dwc3: Disable u1/u2 only for SSP

SNPS STAR: "9001276244: LFPS Handshake Interoperability Issues"
describes two workarounds to avoid interoperability issues with SSP
(Super Speed Plus) USB Hosts. One of the workaround is to disable
U1/U2 completely when operating in SSP Mode.

Change-Id: I64fcd4adc21d985fa68f6cdc42189f75bb1a22d0
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>