Return ENODEV when the selected speculation misfeature is unsupported

When the control of the selected speculation misbehavior is unsupported,
the kernel should return ENODEV according to the documentation:
https://www.kernel.org/doc/html/v4.17/userspace-api/spec_ctrl.html
Current aarch64 implementation of SSB control sometimes returns EINVAL
which is reserved for unimplemented prctl and for violations of reserved
arguments. This change makes the aarch64 implementation consistent with
the x86 implementation and with the documentation.

Signed-off-by: Anthony Steinhauser <asteinhauser@google.com>
Signed-off-by: Will Deacon <will@kernel.org>