V4L/DVB: cx18-alsa: Fix the rates definition and move some buffer freeing code.

Clarify the rates available for the device, and move the freeing of the buffer
to the free routine instead of the close (per Takashi's suggestion).

Thanks to Takashi Iwai for reviewing and providing feedback.

This work was sponsored by ONELAN Limited.

Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>