thunderbolt: Introduce tb_switch_is_icm()

We currently differentiate between SW CM (Software Connection Manager,
sometimes also called External Connection Manager) and ICM (Firmware
based Connection Manager, Internal Connection Manager) by looking
directly at the sw->config.enabled field which may be rather hard to
understand for the casual reader. For this reason introduce a wrapper
function with documentation that should make the intention more clear.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>