MIPS: tracing: Fix the indentation of mcount.S

The commit "MIPS: Tracing: Cleanup the arguments passing of
prepare_ftrace_return" has moved the "jal     prepare_ftrace_return"
instruction after the handling of the 3rd argument but forgot to remove
the superfluous space before the related instructions.
Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1475/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>