V4L/DVB: cx88: Kconfig: Remove EXPERIMENTAL dependency from VIDEO_CX88_ALSA

The cx88-alsa module has been around since January 2006 and has seen no
significant changes since September 2007.  It is stable in operation
and so I believe that the 'experimental' tag is no longer warranted.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>