i2c: Fix the migration warning

While there is a CONFIG_I2C it does not really mean anything and is
defined by only a few dozen boards. This should key off
CONFIG_SYS_I2C_LEGACY instead.

Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>