usb: gadget: Allow up to 900mA current draw for SuperSpeed

Currently 500mA is used as max allowable current to draw over
USB VBUS. However in SuperSpeed a gadget may draw up to 900mA
Update the CONFIG_USB_GADGET_VBUS_DRAW's allowable range to
be up to 900.

Change-Id: I781308065b30b260437f7b3180e276f6a657f504
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>