fit: fixed bug in locate_dtb_in_fit()

If the dtb is the first data of the FIT, the its offset is 0x0. Change the
test to '<' instead of '<='

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>