cmd: smbios: show correct table size for SMBIOS2.1 entry point

The SMBIOS table size for SMBIOS2.1 entry points is in field 'Structure
Table Length' (offset 0x16) and not in field 'Maximum Structure Size'
(offset 0x08).

Rename the receiving variable max_struct_size to table_maximum_size
to avoid future confusion.

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