staging: bcm2835-audio: Use PCM runtime values instead

Some fields in alsa_stream are the values we keep already in PCM
runtime object, hence they are redundant.  Use the standard PCM
runtime values instead of the private copies.

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>