ALSA: firewire-lib: code refactoring for post operation to data block counter

As a result of former commits, post operation to data block count for
cases without CIP_DBC_IS_END_EVENT can be done just with
data_block_counter member of amdtp_stream structure.

This commit adds code refactoring to obsolete local variable for
data block counter.

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