[AVR32] ssize_t should be long, not int

Since size_t is defined as unsigned long, ssize_t ought to be long
and not int. It could have been the other way around, but gcc defines
size_t as unsigned long, so this is correct.

This fixes a couple of printk format warnings.

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