i2c-mv64xxx: correct mv64xxx_i2c_intr() return type

The mv64xxx_i2c_intr() irq handler in drivers/i2c/busses/i2c-mv64xxx.c
is declared as returning 'int', resulting in this compile-time warning:

drivers/i2c/busses/i2c-mv64xxx.c: In function 'mv64xxx_i2c_probe':
drivers/i2c/busses/i2c-mv64xxx.c:540: warning: passing argument 2 of 'request_irq' from incompatible pointer type

Fix: correct the return type to 'irqreturn_t'.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>