of/i2c: call i2c_verify_client from of_find_i2c_device_by_node

i2c_bus_type may contain both I2C clients and I2C adapters. Fix
of_find_i2c_device_by_node to call i2c_verify_client to ensure that it
only returns non-NULL of I2C clients.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>