avr32/lib: fix unaligned memcpy where len < 4

in case of memcpy(p, unaligned, 1..3) we get 1..3 as the
return value instead of p

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>