soc: qcom: pmic_glink: rename callback function pointer

In preparation for subsystem restart (SSR) and protection domain
restart (PDR) where subsystem state transitions would be sent to
PMIC Glink clients via another callback function, change the client
callback function pointer name to msg_cb. This would convey that
this callback is for delivering the messages to a client upon its
registration.

Change-Id: I584d6c5d3898fe54e6928b25502d6382ce6ccbc4
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>