serial: zs: use NULL as a pointer, not 0

struct uart_port::membase is declared as a pointer. So it should be
initialized by NULL, not zero constant.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220421101708.5640-5-jslaby@suse.cz


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