libfdt: fdt_grab_space_(): Fix comparison warning

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

All the involved values cannot be negative, so let's switch the types of
the local variables to unsigned to make the compiler happy.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-4-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>