[PPC] Avoid underflows in NUMA domains

On POWER8 systems with only one memory domain, the "ibm,associativity"
number that corresponds to it is 0, unlike POWER9 systems with two
or more domains, in which the minimum value is 1.

In POWER8 case, subtracting 1 causes an underflow on the unsigned domain
variable and a subsequent index out-of-bounds access.

Reviewed by: jhibbits
Tested by: bdragon, luporl