V4L/DVB: cx18: rename cx18-alsa.c

Rename cx18-alsa.c to cx18-alsa-main.c so that we can call the final .ko file
cx18-alsa.ko
Signed-off-by: NDevin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>