ALSA: firewire-tascam: enclose identifiers referred by single function

Some identifiers are referred just by one functions. In this case, they
can be put into the function definition. This brings two merits; readers
can easily follow codes related to the identifiers, developers are free
from name conflict.

This commit moves such identifiers to each function definition.

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