soc: qcom: pmic_glink: Verify the message length

Currently, data received from the remote subsystem is delivered
to the clients as-is if they've registered for receiving it. This
can cause unintended extraneous buffer reads if the length of the
received message is too small. Verify that the message length is
at least the PMIC Glink header size.

Change-Id: I3774d9d5331794c509bda89f40f4ead2e0ed3843
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>