fjes: Check for error irq

I find that platform_get_irq() will not always succeed.
It will return error irq in case of the failure.
Therefore, it might be better to check it if order to avoid the use of
error irq.

Fixes: 658d439b ("fjes: Introduce FUJITSU Extended Socket Network Device driver")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>