gpio: mpc8xxx: Prevent NULL pointer deref in demux handler

commit cfadd838(powerpc/8xxx: Fix interrupt handling in MPC8xxx GPIO
driver) added an unconditional call of chip->irq_eoi() to the demux
handler.

This leads to a NULL pointer derefernce on MPC512x platforms which use
this driver as well.

Make it conditional.

Reported-by: Thomas Wucher <thwucher@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Felix Radensky <felix@embedded-sol.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: stable@vger.kernel.org
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>