[ARM] pxa: fix one-shot timer mode

One-shot timer mode on PXA has various bugs which prevent kernels
build with NO_HZ enabled booting.  They end up spinning on a
permanently asserted timer interrupt because we don't properly
clear it down - clearing the OIER bit does not stop the pending
interrupt status.  Fix this in the set_mode handler as well.

Moreover, the code which sets the next expiry point may race with
the hardware, and we might not set the match register sufficiently
in the future.  If we encounter that situation, return -ETIME so
the generic time code retries.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>