staging: bcm2835-audio: Simplify PCM creation helpers

All three functions to create PCM objects are fairly resemble, and can
be unified to a single common helper.

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>