usb: host: skip xHCI power management upon port suspend

xHC gets powered down when host mode is stopped. xHCI power
management is intended for ACPI based power management which
is defined for PC architecture. Hence there is no need to
clear run/stop bit and save the runtime register states upon
port suspend.

Change-Id: I9d0b31021e7c47084cd92c8c70e17cff3d057655
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>