libfdt: fdt_resize(): Fix comparison warning

With -Wsign-compare, compilers warn about a mismatching signedness
in a comparison in fdt_resize().

A negative buffer size will surely do us no good, so let's rule this
case out first.
In the actual comparison we then know that a cast to an unsigned type is
safe.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-10-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>