powerpc/ftrace: Use pr_fmt() to namespace error messages

The printks() in our ftrace code have no prefix, so they appear on the
console with very little context, eg:

  Branch out of range

Use pr_fmt() & pr_err() to add a prefix. While we're at it, collapse a
few split lines that don't need to be, and add a missing newline to one
message.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>