USB: dwc3: ep0.c: Disable U1 and U2 low power modes

Currenlty U1 and U2 low power modes are allowed in device mode.
Allowing U1 and U2 low power modes during data transfers in
device mode is causing lot of low level usb bus errors. These
errors will ultimately result in reenumeration during MSC and
RMNET data transfers. Hence disable U1 and U2 low power modes
for now till the issue of low level bus errors is resolved.

CRs-Fixed: 462012
Change-Id: Ic4502a81d572ad130c6fa4ceb2f4befc4f89b3bf
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>