This patch replaces the awkwardly named usb_inc_dev_use() and
usb_dec_dev_use() with usb_get_dev() and usb_put_dev() to match the
naming convention of the rest of the kernel's reference counted
structures.  It also does away with the special case of usb_free_dev(),
and has usb_put_dev() be the same thing (through a #define, just like
usb_free_urb() works.)

Now when the last person calls usb_put_dev() or usb_free_dev() the
structure is cleaned up.  This allows the different host controller
drivers to implement their logic differently if they want to (as they
do), and everyone can be happy and stop arguing about the "proper" way
to write their host controller drivers :)