linux-hdmi-cec: fix remove callback in example driver

We were casting the callback with __devexit_p which is no longer a valid
use case.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
linux-hdmi-cec: add Makefile rules to build modules

Allow building the modules directly from the repository. Path to local
headers are gated with __EXT_BUILD__

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>