net: wangxun: fix kernel panic due to null pointer

When the device uses a custom subsystem vendor ID, the function
wx_sw_init() returns before the memory of 'wx->mac_table' is allocated.
The null pointer will causes the kernel panic.

Fixes: 79625f45

 ("net: wangxun: Move MAC address handling to libwx")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>