usb: sl811-hcd: Add PM_EVENT_POWEROFF into suspend callbacks

usb: sl811-hcd: Add PM_EVENT_POWEROFF into suspend callbacks

When the PM core uses hibernation callbacks for shutdown drivers
will receive PM_EVENT_POWEROFF and should handle it the same as
PM_EVENT_HIBERNATE would have been used.

Tested-by: Eric Naim <dnaim@cachyos.org>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
[ rjw: Changelog adjustment ]
Link: https://patch.msgid.link/20251112224025.2051702-4-superm1@kernel.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>