i2c: omap: switch to platform_get_irq()

that's a nice helper from drivers core which
will give us the exact IRQ number, instead
of a pointer to an IRQ resource.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>