net: systemport: Fix memleak in bcm_sysport_probe

When devm_kcalloc() fails, dev should be freed just
like what we've done in the subsequent error paths.

Fixes: 7b78be48

 ("net: systemport: Dynamically allocate number of TX rings")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>