rpmsg: rpm-smd: Add null check for irq_chip affinity callback

Add the null checks for irq_chip callback while calling
affinity call of glink interrupt.

Change-Id: I62cec6ade0233094ef9a821185677b703d3ba1a3
Signed-off-by: Raghavendra Kakarla <rkakarla@codeaurora.org>