i2c: mpc: Fix ISR return value

ISR should not return IRQ_HANDLED for not handling anything.
This patch fixes the return value of ISR for the same case.

Signed-off-by: Amit Singh Tomar <amit.tomar@freescale.com>
Acked-by: Danielle Costantino <danielle.costantino@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>