USB: cdc-acm: make bitfields unsigned

Fix usb/class sparse warnings:

drivers/usb/class/cdc-acm.h:128:34: error: dubious one-bit signed bitfield
drivers/usb/class/cdc-acm.h:129:24: error: dubious one-bit signed bitfield

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>