media: em28xx: Clean up ALSA PCM API usages

media: em28xx: Clean up ALSA PCM API usages

With the recent change in ALSA PCM core, the whole open-coded vmalloc
buffer handling in this driver can be dropped by replacing with the
managed buffer allocation.

Also, snd_em28xx_hw_capture_free() is dropped since the check of
stream_started flag makes no sense; hw_free callback is guaranteed to
be called after the stream gets stopped.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>