drivers: net: smc911x: 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: ae150435

 ("smsc: Move the SMC (SMSC) drivers")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>