acm: Fix oops when closing ACM tty device right after open has failed.

This commit 10077d4a

 has stopped
checking if there was a valid acm device associated to the tty, which is
not true right after open fails and tty subsystem tries to close the
device.

As an example, open fails with a non-existing device, when probe has
never been called, because the device has never been plugged. This is
common in systems with static modules and no udev.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>