media: cx18: 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.

Cc: Andy Walls <awalls@md.metrocast.net>
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>