oe-selftest: fitimage: fix req_its_fields last field_index

oe-selftest: fitimage: fix req_its_fields last field_index

The last enrty of req_its_fields was not taken into account and the test
passed even if the last entry was not found in the its file.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>