sfc: fix NULL dereferences in ef100_process_design_param()

In the Linux kernel, the following vulnerability has been resolved: sfc: fix NULL dereferences in ef100_process_design_param() Since cited commit, ef100_probe_main() and hence also ef100_check_design_params() run before efx->net_dev is created; consequently, we cannot netif_set_tso_max_size() or _segs() at this point. Move those netif calls to ef100_probe_netdev(), and also replace netif_err within the design params code with pci_err.