net: defxx: Fix missing err handling in dfx_init()

When eisa_driver_register() or tc_register_driver() failed,
the modprobe defxx would fail with some err log as follows:

 Error: Driver 'defxx' is already registered, aborting...

Fix this issue by adding err hanling in dfx_init().

Fixes: e89a2cfb ("[TC] defxx: TURBOchannel support")
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>