MIPS: Don't write ones to reserved entryhi bits.

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: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/mmu_context.h

@@ -165,12 +165,12 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	 * having ASID_MASK smaller than the hardware maximum,
 	 * make sure no "soft" bits become "hard"...
 	 */
-	write_c0_entryhi((read_c0_entryhi() & ~HW_ASID_MASK)
-			| (cpu_context(cpu, next) & ASID_MASK));
+	write_c0_entryhi((read_c0_entryhi() & ~HW_ASID_MASK) |
+			 cpu_asid(cpu, next));
 	ehb(); /* Make sure it propagates to TCStatus */
 	evpe(mtflags);
 #else
-	write_c0_entryhi(cpu_context(cpu, next));
+	write_c0_entryhi(cpu_asid(cpu, next));
 #endif /* CONFIG_MIPS_MT_SMTC */
 	TLBMISS_HANDLER_SETUP_PGD(next->pgd);
 
@@ -226,11 +226,11 @@ activate_mm(struct mm_struct *prev, struct mm_struct *next)
 	}
 	/* See comments for similar code above */
 	write_c0_entryhi((read_c0_entryhi() & ~HW_ASID_MASK) |
-	                 (cpu_context(cpu, next) & ASID_MASK));
+	                 cpu_asid(cpu, next));
 	ehb(); /* Make sure it propagates to TCStatus */
 	evpe(mtflags);
 #else
-	write_c0_entryhi(cpu_context(cpu, next));
+	write_c0_entryhi(cpu_asid(cpu, next));
 #endif /* CONFIG_MIPS_MT_SMTC */
 	TLBMISS_HANDLER_SETUP_PGD(next->pgd);