dvb-frontends: use IS_REACHABLE() instead of IS_ENABLED()

Changeset 9b174527 added this new macro, ensuring that it
is true only if the function is actually reachable.

However, newer drivers were added since when it was written.
So, change those drivers to also use it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>