smbios: correctly name Structure Table Maximum Size field

In the SMBIOS 3 entry point the Structure Table Maximum Size field was
incorrectly named max_struct_size. A Maximum Structure Size field only
exists in the SMBIOS 2.1 entry point and has a different meaning.

Call the Structure Table Length field table_maximum_size.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>