powerpc: Remove variable ‘path’ since not used

In commit eab00a20

 ("powerpc: Move `path` variable inside
DEBUG_PROM") DEBUG_PROM sentinels were added to silence a warning
(treated as error with W=1):

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

Rework the original patch and simplify the code, by removing the
variable ‘path’ completely. Fix line over 90 characters.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>