i2c: Set default I2C bus number

This patch allows using i2c commands (e.g. "i2c probe", "i2c md", etc)
without the need to first select the bus number with e.g. "i2c dev 0".

This is the "i2c" command behavior similar to the one from pre DM, where
by default bus 0 was immediately accessible.

Signed-off-by: Lukasz Majewski <lukma@denx.de>