[PARISC] [MUX] Mux driver bug fix

This patch addresses the problems identified by Russell King in the
following email:

http://lists.parisc-linux.org/pipermail/parisc-linux/2005-December/027912.html



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