xhci: plat: Avoid xhci_resume if skip_resume flag is set

The flag skip_resume is set when DWC3 is already runtime
suspended and prepare it for PM suspend. But if PM suspend of
DWC is not called because of some reason, then directly the PM
resume of xhci-plat is called which invokes xhci_resume when DWC
is in Low Power Mode. This leads to watchdog timeout.
Fix this issue by not calling xhci_resume if skip_resume is set.

Change-Id: I47c946c4491015edcf00602d5a525bf35e7c67ed
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>