qseecom: Validate pointer offset in qseecom_send_modfd_cmd

Validate cmd_req_buf pointer offset in qseecom_send_modfy_cmd, and
make sure cmd buffer address to be within shared bufffer.

Signed-off-by: Zhen Kong <zkong@codeaurora.org>

Change-Id: I431511a92ab2cccbc2daebc0cf76cc3872689a97
Signed-off-by: Zhen Kong <zkong@codeaurora.org>
Signed-off-by: Vignesh Veeramani <quicvignes@codeaurora.org>