serial: 8250: Do nothing if nr_uarts=0

When nr_uarts was set to 0 (via config or 8250_core.nr_uarts), we crash
early on x86 because serial8250_isa_init_ports dereferences base_ops
which remains NULL. In fact, there is nothing to do for all the callers
of serial8250_isa_init_ports if there are no uarts.

Based on suggestions by Peter Hurley.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>