USB Serial Sierra: Dynamic interface detection

This patch changes the method by which the number of ports per interface is
assigned so that it is more dynamic and calculated on the fly (as opposed to
hard coding it). This will allow for faster and easier addition of products.

Signed-off-by: Kevin Lloyd <klloyd@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>