i2c: i2c-gpio: Correctly handle new {sda, scl}-gpios bindings

gpio_request_list_by_name() returns the number of gpios requested.
Notably it swallows the underlying -ENOENT when the "gpios" property
does not exist.

Update the i2c-gpio driver to check for ret == 0 before trying the new
sda-gpios/scl-gpios properties.

Signed-off-by: Chris Packham <judge.packham@gmail.com>