cmd_i2c: Fix help for CONFIG_I2C_CMD_TREE && !CONFIG_I2C_MULTI_BUS

Original code displayed:
 => help i2c
 i2c i2c speed [speed] - show or set I2C bus speed
 i2c md chip address[.0, .1, .2] [# of objects] - read from I2C device
 ...

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>