AVR32: Fix thinko in generic_find_next_zero_le_bit()

The existing implementation of this function seems to be looking for
a one although it should be looking for a zero. This causes trouble
for the ext2 filesystem, which tends to report -ENOSPC without this
patch.

Fix this by complementing each word before scanning.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>