The ACM_READY() macro doesn't seem to do anything useful, and it may
prevent tty_wait_until_sent() from working properly when called from
close.

Previously, acm_tty_chars_in_buffer() returned 0 whenever
acm->port.count was 0. This means close() could return before all the
data has actually been written.
Signed-off-by: NHavard Skinnemoen <hskinnemoen@google.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>