media: cx231xx: Add second frontend option

Include ability to add a second dvb attach style frontend to cx231xx
USB bridge. All current boards set to use frontend[0]. Changes are
backwards compatible with current behaviour.

[mchehab@s-opensource.com: fix some coding style issues]
Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>