powerpc: Move `path` variable inside DEBUG_PROM

Add gcc attribute unused for two variables. Fix warnings treated as errors
with W=1:

  arch/powerpc/kernel/prom_init.c:1388:8: error: variable ‘path’ set but not used [-Werror=unused-but-set-variable]

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>