ethtool: Fix an error code in cxgb2.c

When adapter->registered_device_map is NULL, the value of err is
uncertain, we set err to -EINVAL to avoid ambiguity.

Clean up smatch warning:
drivers/net/ethernet/chelsio/cxgb/cxgb2.c:1114 init_one() warn: missing
error code 'err'

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>