i2c-mpc: drop NO_IRQ

Drop NO_IRQ as 0 is the preferred way to describe 'no irq'
(http://lkml.org/lkml/2005/11/21/221

). This change is safe, as the driver is
only used on powerpc, where NO_IRQ is 0 anyhow.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Ben Dooks <ben-linux@fluff.org>