thunderbolt: Introduce tb_port_next_cap()

This function is useful for walking port config space (adapter)
capability lists. Convert the tb_port_find_cap() to use this as well.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>