[media] solo6x10: Simplify solo_enum_ext_input

Additionally, now it specifies which channels it's showing.

Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>