tpm: acpi: Check eventlog signature before using it

Check the eventlog signature before using it. This avoids using an
empty log, as may be the case when QEMU created the ACPI tables,
rather than probing the EFI log next. This resolves an issue where
the EFI log was empty since an empty ACPI log was used.

Cc: stable@vger.kernel.org
Fixes: 85467f63

 ("tpm: Add support for event log pointer found in TPM2 ACPI table")
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>