avr32: Replace __get_cpu_var with __this_cpu_write

Replace the single use of __get_cpu_var in avr32 with
__this_cpu_write.

Cc: Haavard Skinnemoen <hskinnemoen@gmail.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>