qseecom: Fix issues with qssecom_remove/exit APIs

Clean-up the qseecom_remove() function by removing the null pointer
dereference. Change the logic by removing the unnecessary code and
also modified the qseecom_exit() function with proper calls.

Change-Id: Ibcca1b3c2c37024661505de2550e202b507703aa
Signed-off-by: AnilKumar Chimata <anilc@codeaurora.org>
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>