m68k: Switch do_timer() to xtime_update()

xtime_update() properly takes the xtime_lock

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Cc: Sam Creasey <sammy@sammy.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: johnstul@us.ibm.com
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: hch@infradead.org
Cc: yong.zhang0@gmail.com
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Ungerer <gerg@uclinux.org>
LKML-Reference: <20110127150006.23248.71790.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>