char: lp: introduce list to save port number

When we are registering lp in LP_PARPORT_AUTO mode, we are not keeping
any record of the parallel port number to which lp is connecting.
Add an array to save the port number to it.

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