[MIPS] Fix mprotect() syscall for MIPS32 w/36-bit physical address support

    
Fix mprotect() syscall for MIPS32 CPUs with 36-bit physical address
support: pte_modify() macro didn't clear the hardware page protection bits
before modifying...
    
Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>