staging: comedi: adv_pci1710: define the mux control register bits

For aesthetics, define some macros to set the bits in the mux control
register. Also, rename the 'mux_ext' member of the private data.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>