powerpc/ftrace: Simplify expected_nop_sequence()

Avoid ifdefs around expected_nop_sequence().

While at it make it a bool.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/305d22472f1f92127fba09692df6bb5d079a8cd0.1652074503.git.christophe.leroy@csgroup.eu