[media] dvb uapi docs: enums are passed by value, not reference

Since 2015, the documentation for FE_DISEQC_SEND_BURST, FE_SET_TONE
and FE_SET_VOLTAGE are incorrectly saying that the enums are passed
by reference. They aren't: they're passed by value.

Fix the documentation to reflect reality.

Fixes: 81959d99 ("[media] DocBook: better document FE_DISEQC_SEND_BURST ioctl")
Fixes: d6b6d346 ("[media] DocBook: better document FE_SET_VOLTAGE ioctl")
Fixes: 6dc59e7a ("[media] DocBook: better document FE_SET_TONE ioctl")
Reported-by: Thierry Lelegard <thierry.lelegard@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>