usb: xhci-plat: Disable runtime PM during remove

This is to ensure runtime PM callbacks can't race with
driver removal. Otherwise a runtime suspend or resume
could happen while the HCDs are being removed and result
in a NULL pointer dereference during xhci_suspend/resume.

CRs-fixed: 627204
Change-Id: Ie18497b31499b2c08470d5ed09742e1373e27db6
Signed-off-by: Jack Pham <jackp@codeaurora.org>