arm: omap: gpio: don't access irq_desc array directly

Instead of accessing the irq_desc array directly
we can use irq_to_desc(irq). That will allow us to,
if wanted, select SPARSE_IRQ and irq_descs will be
added to a radix tree, instead of a array.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>