soc: qcom: smd: Make the data parameter to smd_write_segment() const

Some clients would like to pass const data buffers to smd_write_segment().
Change the function signature to permit this usecase.

Change-Id: I96a8012611d79ec9333576d5b5ba0f30657b184d
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>