ACPI: APEI: EINJ: Fix einj_dev release leak

The platform driver conversion of EINJ mistakenly used
platform_device_del() to unwind platform_device_register_full() at
module exit. This leads to a small leak of one 'struct platform_device'
instance per module load/unload cycle. Switch to
platform_device_unregister() which performs both device_del() and final
put_device().

Fixes: 5621fafa

 ("EINJ: Migrate to a platform driver")
Cc: 6.9+ <stable@vger.kernel.org> # 6.9+
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ben Cheatham <Benjamin.Cheatham@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>