[MIPS] Use USECS_PER_SEC / HZ instead of tick_usec in do_gettimeofday.

    
The 'tick_usec' is USER_HZ period in usec.  do_gettimeofday() should
use kernel HZ value.
    
Here is a patch for MIPS.  It seems m32r, m68k and sparc have same
problem though their HZ and USER_HZ are same for now.
    
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>