media: dvb_frontends: fix kernel-doc macros

Now, the Kernel checks for kernel_doc format issues.
Weird enough, it didn't get any of those troubles. Shssst!

Well, let's fix it, as a preventive way to avoid having
hundreds of new warnings on some next Linux version.

Tested by adding all files under dvb-frontends that have
"/**" on them.

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