powerpc/ftrace: Use PPC_RAW_xxx() macros instead of opencoding.

PPC_RAW_xxx() macros are self explanatory and less error prone
than open coding.

Use them in ftrace.c

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9292094c9a69cef6d29ee83f435a557b59c45065.1652074503.git.christophe.leroy@csgroup.eu