input: ti_am335x_tsc: return IRQ_NONE if there was no IRQ for us

The previous patch ("input/ti_am335x_tsc: ACK the HW_PEN irq in ISR")
acked the interrupt so we don't freeze if we don't handle an enabled
interrupt source. The interrupt core has a mechanism for this and to get
it work one should only say that it handled an interrupt if it is
actually the case.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>