[PARISC] [MUX] Correctly report the number of available ports

This patch adds a new function to return the actual number
of ports available.  Some of the built-in Muxes return the
number of supported ports, but not all of these port are
available for use.

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