We've silently been relying on the hardware chopping off excess, reserved
ASID bits for no better reason that it saving an instruction.  Because we
already have:

#define cpu_asid(cpu, mm)       (cpu_context((cpu), (mm)) & ASID_MASK)

in <asm/mmu_context.h>.

We can use a cleanup to avoid writing non-zero bits into the reserved
entryhi bits.  This avoid triggering some debugging assertion in the
Cavium simulator.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>