ARM: OMAP1: ams-delta-fiq: Fix missing irq_ack

Non-serio path of Amstrad Delta FIQ deferred handler depended on
irq_ack() method provided by OMAP GPIO driver.  That method has been
removed by commit 693de831 ("gpio: omap: remove irq_ack method").
Remove useless code from the deferred handler and reimplement the
missing operation inside the base FIQ handler.

Should another dependency - irq_unmask() - be ever removed from the OMAP
GPIO driver, WARN once if missing.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>