USB: allow users to run setserial with cdc-acm

We had a user report that running setserial on /dev/ttyACM0 didn't work.
He pointed at an old patch by Oliver Neukum from 2008 that never went anywhere..
http://permalink.gmane.org/gmane.linux.usb.general/9236

I made some minor changes to get it to apply again, and got the user to retest on 3.1,
and he reported it worked for him. https://bugzilla.redhat.com/show_bug.cgi?id=787607

The diff below is against 3.3rc. The only difference between this and
the version the user tested is the removal of the if (!ACM_READY) test
Havard removed ACM_READY in 99823f45


I'm unclear if there's need for a different test in its place.

From: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>