greybus: camera: Improve module registration mechanism

Registering more then one module at same time was not
possible with previous implementation. Also unregistering
of the module was missing leading to many instability issues
when camera module is ejected when camera is still active.

Signed-off-by: Gjorgji Rosikopulos <grosikopulos@mm-sol.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>