'state' variable is only set but never read in amiserial's shutdown.
Drop it.

It was like this since the driver was added to the tree as far as I can
tell.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210714091314.8292-11-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>