efi: Fix efi_power_off() not being run before acpi_power_off() when necessary

Commit 98f30d0e ("ACPI: power: Switch to sys-off handler API")
switched the ACPI sleep code from directly setting the old global
pm_power_off handler to using the new register_sys_off_handler()
mechanism with a priority of SYS_OFF_PRIO_FIRMWARE.

This is a problem when the old global pm_power_off handler would later
be overwritten, such as done by the late_initcall(efi_shutdown_init):

	if (efi_poweroff_required())
		pm_power_off = efi_power_off;

The old global pm_power_off handler gets run with a priority of
SYS_OFF_PRIO_DEFAULT which is lower then SYS_OFF_PRIO_FIRMWARE, causing
acpi_power_off() to run first, changing the behavior from before
the ACPI sleep code switched to the new register_sys_off_handler().

Switch the registering of efi_power_off over to register_sys_off_handler()
with a priority of SYS_OFF_PRIO_FIRMWARE + 1 so that it will run before
acpi_power_off() as before.

Note since the new sys-off-handler code will try all handlers in
priority order, there is no more need for the EFI code to store and
call the original pm_power_off handler.

Fixes: 98f30d0e ("ACPI: power: Switch to sys-off handler API")
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220708131412.81078-3-hdegoede@redhat.com