soc: qcom: rpmh: check return value of __fill_rpmh_msg

It is possible that number of commands to be send can exceed
MAX_RPMH_PAYLOAD.

This change adds error checking for such case.

Change-Id: I9c05bf3befd6590fee4625230bb44d7711c625a7
Signed-off-by: Tushar Nimkar <tnimkar@codeaurora.org>