efi_loader: sanitize efi_tcg2_final_events_table definition

The length of the variable name typically is not 1.
Neither the length of the variable name nor the size of the appended
data is known in the include.

* Define the size of element variable_name as variable.
* Remove the unusable element variable_data.

Addresses-Coverity-ID: 467400 Out-of-bounds read
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>