cmd_i2c: Fix i2c help command output when CONFIG_I2C_MUX

When CONFIG_I2C_MUX was defined the output of 'help i2c' was not
correct, eg:

=> help i2c
i2c bus [muxtype:muxaddr:muxchannel] - add a new bus reached over muxes.
speed [speed] - show or set I2C bus speed
i2c dev [dev] - show or set current I2C bus
...

It has been changed to:
i2c speed [speed] - show or set I2C bus speed
i2c bus [muxtype:muxaddr:muxchannel] - add a new bus reached over muxes
i2c dev [dev] - show or set current I2C bus
...

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