qseecom: Move checks to start of __qseecom_send_cmd()

data pointer validation is done after dereferencing it which might
throw an exception. This patch moves the data pointer validation
checks to starting of the function.

Change-Id: I12d201e53ad1d3098bf45c427553b048ca4d8ac0
Signed-off-by: AnilKumar Chimata <anilc@codeaurora.org>