media: dvbdev: fix building on ia64

Not sure why, but, on ia64, with Linaro's gcc 7.3 compiler,
using #ifdef (CONFIG_I2C) is not OK.

So, replace it by IS_ENABLED(CONFIG_I2C), in order to fix the
builds there.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>