ALSA: firewire-tascam: use better name for local variables to describe their intension

In the callback function of asynchronous MIDI port, the intension of some
local variables are not clear.

This commit improves them. The 'len' variable is used to calculate the
number of MIDI bytes including in the transaction. The 'consume' variable
is used to return the actual number of consumed bytes in ALSA MIDI buffer.

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