efi: stmm: Drop unneeded null pointer check

The API documenation of setup_mm_hdr does not mention that dptr can be
NULL, this is a local function, and no caller passes NULL. So drop the
unneeded check.

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>