xkb: Fix parsing of XkbSA_DeviceValuator action type

The xkb spec defines that the 7th element of the DeviceValuator key
action description is "valuator 2 value".

This error most likely was accidentally introduced as a copy-paste error
in edeb033f.

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>