xhci-plat: Use correct dev to get usb-phy handle

Current code is setting sysdev pointing to USB glue device. This leads
to failure of getting usb-phy handle as incorrect device is passed to
get handle. Hence fix it by using correct dev pointer to get handle
to usb-phy.

Change-Id: Ia7ea49b25b346c308815e83ecbcbe05fe701c4cd
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>