Bluetooth: controller: Support SMI TX flag as setting

SMI TX is different than other controller features in that it does not
necessarily imply any software changes; whether SMI TX is supported
may be simply a matter of hardware calibration. This change supports
using the same software on chips that do or do not support SMI TX
depending on calibration.

Signed-off-by: Wolfgang Puffitsch <wopu@demant.com>