thunderbolt: Do not start firmware unless asked by the user

Since now we can do pretty much the same thing in the software
connection manager than the firmware would do, there is no point
starting it by default. Instead we can just continue using the software
connection manager.

Make it possible for user to switch between the two by adding a module
pararameter (start_icm) which is by default false. Having this ability
to enable the firmware may be useful at least when debugging possible
issues with the software connection manager implementation.

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