firmware: ti_sci: fix the secure_hdr in do_xfer

The ti_sci driver in U-Boot has support for secure_msg as part of it's
do_xfer function. This let's U-boot send secure messages during boot up.

The protocol to send such secure messages is described as part of the
struct ti_sci_secure_msg_hdr. As part of this, there are 2 fields for
checksum and reserved that occupy the first 4 bytes of any secure
message. This is called as the secure_hdr.

As of now, the secure_hdr needs to be 0 init-ed before sending secure
messages. However the existing code was never putting the zero-inited vars
into the secure_buf, leading to possibility of the first 4 bytes of
secure_buf being possibly garbage.

Fix this by initialising the secure_hdr itself to the secure_buf
location, thus when we make secure_hdr members 0, it automatically ensures
the first 4 bytes of secure_buf are 0.

Fixes: 32cd2512 ("firmware: Add basic support for TI System Control Interface (TI SCI)")
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dhruva Gole <d-gole@ti.com>