amiserial: use memset to zero serial_state

Zeroing each member of struct serial_state in probe is fragile and
overly complicated. Do one memset for them all.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210714091314.8292-6-jslaby@suse.cz


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