i2c: stub: use pr_fmt

Instead of hard coding "i2c-stub:", let's use the pr_fmt mechanism to
achieve the same more easily. This makes it easier to stay consistent
when adding new messages. Also, remove an unneeded OOM message while we
are here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>