KVM: PPC: Book3S HV: Return error from h_set_mode(SET_DAWR) on POWER9

Return H_P2 on a h_set_mode(SET_DAWR) on POWER9 where the DAWR is
disabled.

Current Linux guests ignore this error, so they will silently not get
the DAWR (sigh). The same error code is being used by POWERVM in this
case.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>