thunderbolt: Avoid reserved fields in path config space for USB4 routers

thunderbolt: Avoid reserved fields in path config space for USB4 routers

According to USB4 spec, USB4 Connection Manager shall not change value
of any fields that are defined as "RsvdZ" or "VD".  Specifically fields:
Path Credits Allocated, IFC, ISE fields in path config space shall not
be written by CM. To handle this, CM shall first read current path
config space from the hardware, change only the fields that can be
changed, and then write back the path config space.

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