media: cec/core/cec-notifier: use IS_REACHABLE(CONFIG_I2C)

If CONFIG_I2C=m and CONFIG_CEC_CORE=y then of_find_i2c_device_by_node()
is not reachable. So use IS_REACHABLE instead of IS_ENABLED.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: a62943c145c1 ("media: cec-notifier: also search for HDMI devices on I2C")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>