ARM: OMAP2+: remove incorrect irq_chip ack field

Each irq_chip for the main interrupt controller has offsets set for irq
masking registers, which added to respective base results in a pointer
to appropriate hardware register. However this is not correct for
INTC_CONTROL as there is only one INTC_CONTROL register. This does not
cause problems because generic ack code is never called, but remove
this assignment to avoid confusion.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>