alpha: Change do_timer() to xtime_update()

xtime_update() takes the xtime_lock itself.

timer_interrupt() is only called on the boot cpu. See do_entInt(). So
"state" in timer_interrupt does not require protection by xtime_lock.

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: johnstul@us.ibm.com
Cc: hch@infradead.org
Cc: yong.zhang0@gmail.com
LKML-Reference: <20110127145915.23248.20919.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>