net: ks8851: Check for error irq

Because platform_get_irq() could fail and return error irq.
Therefore, it might be better to check it if order to avoid the use of
error irq.

Fixes: 797047f8

 ("net: ks8851: Implement Parallel bus operations")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>