ALSA: tlv: compute TLV_*_ITEM lengths automatically

Add helper macros with a little bit of preprocessor magic to
automatically compute the length of a TLV item.  This lets us avoid
having to compute this by hand, and will allow to use items that do
not use a fixed length.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>