net/smsc911x: Fix deferred probe for interrupt

The interrupt handler may not be available when smsc911x probes if the
interrupt handler is a GPIO controller for example. Let's fix that
by adding handling for -EPROBE_DEFER.

Cc: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>