powerpc/vdso: Remove vdso_ready

There is no way to get out of vdso_init() prematuraly anymore.

Remove vdso_ready as it will always be 1.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0e1e18c6329b848aa3edeeba76509b4d76182e7d.1601197618.git.christophe.leroy@csgroup.eu