tcgbios: Only measure size indicated in UEFI partition table header

Remove the reserved byte array from the UEFI_PARTITION_TABLE_HEADER
to avoid measuring zero bytes. Only log and measure the header as
indicated by its size. This size is expected to be exactly the size
of UEFI_PARTITION_TABLE_HEADER (= 92 bytes).

Fixes: 16a1867425ee ("tcgbios: Measure the GPT table")
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>