staging: bcm2835-audio: Fix draining behavior regression

The PCM draining behavior got broken since the recent refactoring, and
this turned out to be the incorrect expectation of the firmware
behavior regarding "draining".  While I expected the "drain" flag at
the stop operation would do processing the queued samples, it seems
rather dropping the samples.

As a quick fix, just drop the SNDRV_PCM_INFO_DRAIN_TRIGGER flag, so
that the driver uses the normal PCM draining procedure.  Also, put
some caution comment to the function for future readers not to fall
into the same pitfall.

Fixes: d7ca3a71 ("staging: bcm2835-audio: Operate non-atomic PCM ops")
BugLink: https://github.com/raspberrypi/linux/issues/2983


Cc: stable@vger.kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>