tools/atmelimage.c: Fix warning when debug is enabled

Otherwise we get:
tools/atmelimage.c:134:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t’ [-Wformat=]
debug("atmelimage: interrupt vector #%d is 0x%08X\n", pos+1,
^

Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>