USB: sierra driver - add update dtr logic

The following improvements were made:
 - Fixed control line issue where asserting DTR on ep5 would close ep2
 - Added support for calc_num_ports (will help support future composite
   devices)
Signed-off-by: Kevin Lloyd <linux@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>