char: lp: detach the device when parallel port is removed

When the parallel port is usb based and the lp attaches to it, we do
get /dev/lp0, but when we remove the usb device and the parallel port
is gone, we are still left with /dev/lp0.
Unregister the device properly in the detach routine based on the port
number it has connected to.

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>