efi: stmm: Drop unused EFI error from setup_mm_hdr arguments

No caller ever evaluates what we return in 'ret'. They only use the
return code of the function.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>