ion: Don't allow building ION as a module.

ION doesn't export the proper symbols for it to be a module. This
causes build issues when ION is configured as a module.

Since Andorid kernels rarely use modules (I think recent policy
requires no modules?), go ahead and set the ION config to a bool
from the tristate option.

If folks decide ION as a module is important, we will have to go
through and export the various needed symbols.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>