drivers: eth_enc424j600: terminate driver thread only when debugging

The controller has several interrupt sources which are signaled
via a single INT pin. Only the interrupt sources that are explicitly
switched on during controller initialization may generate an interrupt
signal. Currently there are only PHY Link Status Change Interrupt
and RX Packet Pending Interrupt enabled. So there is no other reason why
an interrupt can be triggered.

Terminate interrupt handling thread on unknown interrupt
only when debugging, as there are concerns that stopping
thread in the field is going too far.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>