USB: serial: digi_acceleport: reduce chars_in_buffer over-reporting

Due to an ancient quirk in n_tty poll implementation, the
digi_acceleport driver has been reporting that its queue contains 256
(WAKEUP_CHARS) characters whenever its write URB is in use.

This has not been necessary since 2003 when the line-discipline started
taking the write room into account so let's return the maximum transfer
size again in order to over-report a little less and incidentally fix
the related debug statement.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>