sandbox: i2c: search child emul dev and check its uclass id

sandbox: i2c: search child emul dev and check its uclass id

The function get_emul() in sandbox i2c bus driver, always returns
first child as i2c emul device. This may only work for i2c devices
with a single child, which is an only i2c emul device.

In case when i2c device has more than one child (e.g. PMIC), and
one is i2c emul, then the function should search it by check uclass
id for each child. This patch add this change to the get_emul().

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Tested on sandbox:
Tested-by: Simon Glass <sjg@chromium.org>