tee: optee: ffa: Fix missing-field-initializers warning

The 'missing-field-initializers' warning was reported
when building with W=2.
This patch use designated initializers for
'struct ffa_send_direct_data' to suppress the warning
and clarify the initialization intent.

Signed-off-by: ming-jen.chang <ming-jen.chang@mediatek.com>
Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>