cmd: efidebug: add missing efi_free_pool for dh subcommand

This adds the missing efi_free_pool call for dh subcommand.

Fixes: a8014620 ("cmd: efidebug: add dh command")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>