dm: i2c: Make i2c_get_chip_for_busnum() fail if the chip is not detected

i2c_get_chip_for_busnum() really should check the presence of the chip on
the bus. Most of the users of this function assume that this is done.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Heiko Schocher <hs@denx.de>