powerpc/ftrace: Fix typo in mask of opcode

In commit 24a1bdc3, "Fix ABIv2 issues with __ftrace_make_call", Anton
changed the logic that checks for the expected code sequence when
patching a module.

We missed the typo in the mask, 0xffff00000 should be 0xffff0000, which
has the effect of making the test always true.

That makes it impossible to ftrace against modules, eg:

  Unexpected call sequence: 48000008 e8410018
  WARNING: at ../kernel/trace/ftrace.c:1638
  ftrace failed to modify [<d000000007cf001c>] rng_dev_open+0x1c/0x70 [rng_core]

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>