libfdt: fdt_move(): Fix comparison warnings

With -Wsign-compare, compilers warn about a mismatching signedness
in comparisons in fdt_move().

This stems from "bufsize" being passed in as a signed integer, even
though we would expect a buffer size to be positive.

Short of changing the prototype, check that bufsize is not negative, and
cast it to an unsigned type in the comparison.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201001164630.4980-3-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>