lib: fdt: Fix fdtdec_setup_mem..() conversion to livetree API

Repair incorrectly negated condition in the original patch which broke
DT memory node parsing on everything which has more than one DT memory
node, e.g. R-Car3.

In case multiple valid memory nodes are present in the DT, the original
patch would complete parsing cycle for the first memory node, then move
on to the next one, identify it as a valid, and end the parsing. The fix
is to invert the condition, to make the code behave as it did before the
livetree conversion, so it would continue parsing the subsequent memory
nodes as well.

Fixes: c2f0950c ("lib: fdt: Convert fdtdes_setup_mem..() to livetree API")
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Tested-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>