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

[ Upstream commit e5f5e380e0f3bb11f04ca5bc66a551e58e0ad26e ]

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>