powerpc/ftrace: Fix confusing help text for DISABLE_MPROFILE_KERNEL

The final paragraph of the help text is reversed. We want to enable
this option by default, and disable it if the toolchain has a working
-mprofile-kernel.

Fixes: 8c50b72a

 ("powerpc/ftrace: Add Kconfig & Make glue for mprofile-kernel")
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>