arm64: kexec_file: Refactor setup_dtb() to consolidate error checking

setup_dtb() is a little difficult to read. This is largely because it
duplicates the FDT -> Linux errno conversion for every intermediate
return value, but also because of silly cosmetic things like naming
and formatting.

Given that this is all brand new, refactor the function to get us off on
the right foot.

Signed-off-by: Will Deacon <will.deacon@arm.com>