selftests/powerpc: Fix subpage_prot test to return !0 on failure

It's helpful for automated testing if the test returns error codes back
to the calling program.

While we're here fix all the usages of %p to remove the double 0x, ie.
%p already includes 0x.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>