[PARISC] [MUX] Make the Serial Mux driver work as module

The following updates are based off a patch from willy:
  * Removal of the mux_card list.
  * Add the mux_remove function.

Other updates:
  * Re-organize the driver structure a bit to make the
    mux_init and mux_exit functions more symmetrical.
  * Added the del_timer.
  * Unregistered the console.

At this point I can insmod, rmmod, and re-insmod the
mux without any failures.

Signed-off-by: Ryan Bradetich <rbrad@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>