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>

common/cmd_i2c.c

@@ -1301,10 +1301,10 @@ int do_i2c(cmd_tbl_t * cmdtp, int flag, int argc, char *argv[])
 U_BOOT_CMD(
 	i2c, 6, 1, do_i2c,
 	"I2C sub-system",
+	"speed [speed] - show or set I2C bus speed\n"
 #if defined(CONFIG_I2C_MUX)
-	"bus [muxtype:muxaddr:muxchannel] - add a new bus reached over muxes.\n"
+	"i2c bus [muxtype:muxaddr:muxchannel] - add a new bus reached over muxes\n"
 #endif  /* CONFIG_I2C_MUX */
-	"speed [speed] - show or set I2C bus speed\n"
 #if defined(CONFIG_I2C_MULTI_BUS)
 	"i2c dev [dev] - show or set current I2C bus\n"
 #endif  /* CONFIG_I2C_MULTI_BUS */