Input: altera_ps2 - use correct type for irq return value

The irq function altera_ps2_rxint returns an irqreturn_t, so use the
same type for variable storing the return value.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>