freescale: fix long help handling in mac command

CONFIG_SYS_LONGHELP=n we want to reduce the size of the U-Boot binary.
The long text should be reduced to and empty string in this case.

There is not need to call cmd_usage() directly. It is sufficient to
return CMD_RET_USAGE.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>