amiserial: remove unused state from shutdown

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