gianfar: add missing iounmap() on error in gianfar_ptp_probe()

Add the missing iounmap() before return from gianfar_ptp_probe()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>