ALSA: bcm2835: defer bcm2835 PCM write using the workqueue

Only PCM playback and stop are deferred using a workqueue on
the ALSA bcm2835 driver. The actual writing of PCM samples
is synchronous.

This works well for the read/write transfer method since
the snd_pcm_ops .copy function pointer runs on process context.

But the snd_pcm_ops .ack function pointer used to implement
direct read/write transfer using a memory-mapped I/O, runs
with interrupts disabled. Since the Kernel to VideoCore
interface driver has to be able to sleep, PCM write has to be
deferred to in preparation to add mmap support to the driver.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>