usb: host: xhci: Add helper function to return controller id

Function provides controller id used by a remote entity
to identify which usb controller to program to initiate
data transfer.

Change-Id: Ie700363c1a4a19aeb6b51305c97298f3b4d6e387
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>