staging: bcm2835-audio: Remove superfluous open flag

All the alsa_stream->open flag checks in the current code are
redundant, and they cannot be racy.  For the code simplification,
let's remove the flag and its check.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>