[MIPS] time: Fix cevt-r4k.c for 64-bit kernel

The expression "(long)(read_c0_count() - cnt)" can never be a negative
value on 64-bit kernel.  Cast to "int" before comparison.

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