thunderbolt: Make use of SI units from units.h

In a couple of places it seems reasonable to use MEGA intead
of explicit number. It makes code more readable and robust.

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