usb: Auto-load cdc_acm module when device opened.

The cdc_acm module is missing the char-major-166-* alias that would
cause it to be auto-loaded when a device of that type is opened.  This
patch adds the alias.

Signed-off-by: Scott James Remnant <scott@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>