pcm: hw: maintain fallback mode for status data mapping

pcm: hw: maintain fallback mode for status data mapping

In current implementation, results to map status/control data are not
maintained separately. It's handled as a fatal error that mapping of status
data is successful and mapping of control data is failed. However, it's
possible to handle this case by utilizing fallback buffer.

This commit adds a member into a local structure to maintain fallback mode
just for the mapping of status data as a preparation of later commit, in
which mapping results are maintained separately for each of status/control
data.

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