qos: Add NULL checking for irq descriptor

Check for NULL for irq descriptor to prevent deferencing an invalid
address.  The qos value is then ignored as it cannot be associated with
the requested irq.

Change-Id: I82fdc9b10ffff89032a0f0e9dade42e4c670f4b9
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>