soc: qcom: qsee_ipc_irq: Sanitize IRQ desc

There is a chance that the irq_to_desc function will return NULL if the
client unregisters their IRQ. Check if desc is NULL before calling the
IRQ_HANDLER.

Change-Id: I10950e19138eb8e6f7673f61b7de78696e4c2c27
Signed-off-by: Chris Lew <clew@codeaurora.org>