ALSA: firewire-lib: code refactoring for local variables

It's better to use int type for loop index. For consistency, the name
of local variable for the number of data block should be plural.

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