USB: serial: digi_acceleport: fix write-wakeup deadlocks

The driver must not call tty_wakeup() while holding its private lock as
line disciplines are allowed to call back into write() from
write_wakeup(), leading to a deadlock.

Also remove the unneeded work struct that was used to defer wakeup in
order to work around a possible race in ancient times (see comment about
n_tty write_chan() in commit 14b54e39 ("USB: serial: remove
changelogs and old todo entries")).

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>