USB: Support for BT On-Air USB modem in cdc-acm.c

The patch below is a necessary workaround to support the BT On-Air USB modem, which
fails to initialise properly during normal probing thus:

Sep 30 17:34:57 sled kernel: drivers/usb/class/cdc-acm.c: Zero length descriptor references
Sep 30 17:34:57 sled kernel: cdc_acm: probe of 1-1.2:1.0 failed with error -22

Adding the patch below causes the probing section to be skipped, and the modem
then initialises correctly.

Signed-off-by: Chris Malley <mail@chrismalley.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>