powerpc/ftrace: Use size macro instead of opencoding

0x80000000 is SZ_2G. Use it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Fix comparison against unsigned -SZ_2G]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bb6626e884acffe87b58736291df57db3deaa9b9.1652074503.git.christophe.leroy@csgroup.eu