axfer: print help text of command entry

axfer: print help text of command entry

This commit adds help text of command entry. This is printed when
help subcommand is given, or a valid subcommand or a valid direction
are not given.

Unfortunately, at present, execution of alias (aplay/arecord) with help
options prints the added help text. It should print help options in
transfer subcommand. This bug will fixed in future commits.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>