soc: qcom: smd: Drop const marking on writel wrapper

Marking this pointer as const here doesn't make sense as we're
going to be changing the value of what the pointer is pointing
at. If we didn't have RTB logging hooks in place in <asm/io.h>
the compilation would have warned about const being discarded.

Change-Id: I02ec04c52278850a6f82357799183e5f15e974d5
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>