MIPS: Don't clobber CP0_STATUS value for CONFIG_MIPS_MT_SMTC

Reported-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Patchwork: https://patchwork.linux-mips.org/patch/2753/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>