MIPS: Unbreak function tracer for 64-bit kernel.

Commit 58b69401 [MIPS: Function tracer: Fix broken function tracing]
completely broke the function tracer for 64-bit kernels.  The symptom is
a system hang very early in the boot process.

The fix: Remove/fix $sp adjustments for 64-bit case.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: Al Cooper <alcooperx@gmail.com>
Cc: viric@viric.name
Cc: stable@vger.kernel.org # 3.8.x
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>