V4L/DVB (9003): S2API: Remove the DTV_SET_ and DTV_GET_ prefixes

The API now assumes that ioctl calls for FE_SET_PROPERTY and all
set commands, and FE_GET_PROPERTY are get commands.

Simplification.

Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>