powerpc/64e: Simplify address calculation in secondary hold loop

As the earlier comment explains, __secondary_hold_spinloop does not have
to be accessed at its virtual address, slightly simplifying code.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230203113858.1152093-4-npiggin@gmail.com