memory: omap-gpmc: allow probe of child nodes to fail

A recent commit (inadvertently?) changed how failed probe of a gpmc
child node was handled. Instead of proceeding with setting up any other
children as before, a single error now aborts the whole process.

This change broke networking on some Overo boards due to probe failing
for an unrelated nand node. This second issue should obviously be
fixed, but let's restore the old behaviour of allowing child-node
probe to fail to avoid further similar breakage on other systems.

Fixes: d2d00862 ("memory: omap-gpmc: Support general purpose input
for WAITPINs")

Cc: stable@vger.kernel.org  # v4.7+
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>