soc: qcom: smd: Introduce callback setter

Introduce a setter for the callback function pointer to clarify the
locking around the operation and to reduce some duplication.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>