[pinmux/dif] Fix check for output selection arg
[pinmux/dif] Fix check for output selection arg

There are three fixed values (0, 1, Z) for the output mux, not just two.

Signed-off-by: Alexander Williams <d0fb8a15e7b6f5e62278a4cf44515038c232186c@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan