net: smc91x: propagate irq return code

The smc91x driver doesn't honor the probe deferral mechanism when the
interrupt source is not yet available, such as one provided by a gpio
controller not probed.

Fix this by propagating the platform_get_irq() error code as the probe
return value.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>