i2c: don't expose function which is only used internally

i2c_setup_smbus_alert() is only needed within the I2C core, so no need
to expose it to other modules.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>