serial: sifive: set pointer to NULL rather than 0.

Clean up the assignment of sifive_serial_console_ports[ssp->port.line],
this is a pointer to a struct sifive_serial_port so the assignment
should be a NULL rather than a integer 0.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210925224726.183360-1-colin.king@canonical.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>