thunderbolt: Move width_name() helper to tb.h

We are going to use it in subsequent patches, so make it available outside of
switch.c. Also, change the name to tb_width_name() to follow the naming
conventions.

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