usb: roles: intel_xhci: Enable runtime PM

This fixes an issue where the mux does not get configured
when the parent device is suspended. The registers for this
mux are mapped to the parent device MMIO (usually xHCI PCI
device), so in order for the driver to be able to program
the registers, the parent device must be resumed.

Reported-by: Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@intel.com>
Fixes: f6fb9ec0

 ("usb: roles: Add Intel xHCI USB role switch driver")
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>