usb: dwc3: core: Fix broken system suspend/resume on AM437x

On TI's AM437x, the DWC3 controller looses state after a
system suspend/resume. We are re-initializing the controller
but we miss restoring the PRTCAP register. This causes
USB host to break on AM437x after a system suspend/resume.

Fix this by restoring the PRTCAP register on system resume.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>