sbus: Use of_get_child_by_name helper

Use the of_get_child_by_name() helper instead of open coding searching
for the '/options' node. This removes directly accessing the name
pointer as well.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: sparclinux@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>