fdt: Fix FIT header verification in mkimage and conduct same checks as bootm

FIT header verification in mkimage was treating a return code as a boolean,
which meant that failures in validating the fit were seen as successes.

Additionally, mkimage was checking all formats to find a header which
passes validation, rather than using the image type specified to
mkimage.

checkpatch.pl checks for lines ending with '(' and alignment matching
open parentheses are ignored to keep with existing coding style.

Signed-off-by: Jordan Hand <jorhand@microsoft.com>