avr32: remove unneeded cast in atomic.h

This int cast is superfluous since system.h cmpxchg already casts it in
(typeof(*(ptr))).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>