USB: add read support to usb-serial/../new_id

Keep the usb-serial support for dynamic IDs in sync with the usb
support.  This enables readout of dynamic device IDs for
usb-serial drivers.  Common code is exported from the usb core
system and reused by the usb-serial bus driver.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>