powerpc/book3s64/pkeys: Simplify pkey disable branch

Make the default value FALSE (pkey enabled) and set to TRUE when we
find the total number of keys supported to be zero.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200709032946.881753-10-aneesh.kumar@linux.ibm.com