powerpc/fixmap: Fix the size of the early debug area

Commit ("03fd42d4 powerpc/fixmap: Fix FIX_EARLY_DEBUG_BASE when
page size is 256k") reworked the setup of the early debug area and
mistakenly replaced 128 * 1024 by SZ_128.

Change to SZ_128K to restore the original 128 kbytes size of the area.

Fixes: 03fd42d4 ("powerpc/fixmap: Fix FIX_EARLY_DEBUG_BASE when page size is 256k")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/996184974d674ff984643778cf1cdd7fe58cc065.1597644194.git.christophe.leroy@csgroup.eu