staging: comedi: adv_pci1710: fix AI channels 16-31 for PCI-1713

The Advantech PCI-1713 has 32 analog input channels, but an incorrect
bit-mask in the definition of the `PCI171X_MUX_CHANH(x)` and
PCI171X_MUX_CHANL(x)` macros is causing channels 16 to 31 to be aliases
of channels 0 to 15.  Change the bit-mask value from 0xf to 0xff to fix
it.  Note that the channel numbers will have been range checked already,
so the bit-mask isn't really needed.

Fixes: 92c65e55

 ("staging: comedi: adv_pci1710: define the mux control register bits")
Reported-by: Dmytro Fil <monkdaf@gmail.com>
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20191227170054.32051-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>