powerpc/ftrace: Remove nops after the call to ftrace_stub

ftrace_stub is within the same CU, so there is no need for a subsequent
nop instruction.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/8ee5ec520e37d5523654bb2cd65a17512fb774e2.1702045299.git.naveen@kernel.org