thunderbolt: Add helper macro to iterate over switch ports

There are quite many places in the driver where we iterate over each
port in the switch. To make it bit more convenient, add a macro that can
be used to iterate over each port and convert existing call sites to use it.

This is based on code by Lukas Wunner.

No functional changes.

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