net: wan: z85230: drop unused async state

According to the changelog, asynchronous mode was dropped sometime
before v2.2. Let's get rid of the unused driver-specific async state as
well so that it doesn't show up when doing tree-wide tty work.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>