avr32: nmi_enter() without nmi_exit()

While updating the rcu code, I noticed that do_nmi() for AVR32 is odd:
There is an nmi_enter() call without an nmi_exit().
This can't be correct, it breaks rcu (at least the preempt version) and
lockdep.

[haavard.skinnemoen@atmel.com: fixed another case that returned directly]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>