net: tehuti: fix error return code in bdx_probe()

When bdx_read_mac() fails, no error return code of bdx_probe()
is assigned.
To fix this bug, err is assigned with -EFAULT as error return code.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>