soc: qcom: cmd-db: Remove memcpy()ing from cmd_db_get_header()

The cmd_db_get_header() function is a static local function that doesn't
need to copy anything from one place to another. Instead, it can just
point into the region by returning pointers to what we're looking for.
If we do that, we should mark what we're returning as const so that code
can't modify cmd-db without an obvious cast.

Cc: Mahesh Sivasubramanian <msivasub@codeaurora.org>
Cc: Lina Iyer <ilina@codeaurora.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Evan Green <evgreen@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>