efi_loader: factor out the common code from efi_transfer_secure_state()

efi_set_secure_stat() provides the common code for each stat transition
caused by efi_transfer_secure_state().

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Correct description of return value.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>