selftests/powerpc: Use stderr for debug messages in eeh-functions

We want to use stdout to return lists of devices, etc so log debug / status
messages to stderr rather than stdout.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201103044503.917128-2-oohall@gmail.com