usb: dwc3: Fix offset for register read in dwc3_masked_write_readback

commit 375bf771 ("usb: dwc3: Add API to read back after register
write") API is using wrong offset from xHCI base address. Offset needs
to start from global address space. Fix this by subtracting DWC3 global
address start offset from offset received from API parameter.

Change-Id: If3ed2a9d5047b2deaaae0c9c73049768d039cff9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>