tpm/tpm_i2c_stm_st33: Fix potential bug in tpm_stm_i2c_send

When sending data in tpm_stm_i2c_send, each loop iteration send buf.
Send buf + i instead as the goal of this for loop is to send a number
of byte from buf that fit in burstcnt. Once those byte are sent, we are
supposed to send the next ones.

The driver was working because the burstcount value returns always the maximum size for a TPM
command or response. (0x800 for a command and 0x400 for a response).

Cc: stable@vger.kernel.org

Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>