[MIPS] unexport null_perf_irq() and make it static

This patch unexports the null_perf_irq() symbol, and simultaneously
makes this function static.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
3 files changed