rocker: call correct unregister function on error

Use the correct unregister function matching the register
function on the error path.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Fixes: c1beeef7 ("rocker: implement IPv4 fib offloading")
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>