xtensa: Switch do_timer() to xtime_update()

xtime_update() takes the xtime_lock itself.

set_linux_timer() does not need to be protected by xtime_lock.

[ tglx: This code is broken on SMP anyway. ]

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Cc: Chris Zankel <chris@zankel.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: johnstul@us.ibm.com
Cc: hch@infradead.org
Cc: yong.zhang0@gmail.com
LKML-Reference: <20110127150027.23248.61798.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>