thunderbolt: Add dummy read after port capability list walk on Light Ridge

Light Ridge has an issue where reading the next capability pointer
location in port config space the read data is not cleared. It is fine
to read capabilities each after another so only thing we need to do is
to make sure we issue dummy read after tb_port_find_cap() is finished to
avoid the issue in next read.

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