MIPS: TXx9: Fix possible overflow in clock calculations

Addition of -fwrapv option in 2.6.29 discloses possible overflow with
signed arithmetics.  For example, result of "a * 6 / 12" (int a =
400000000) is 200000000 without -fwrapv but -157913941 with -fwrapv.

Change some variable to unsigned to avoid such overflows.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>