thunderbolt: Introduce tb_switch_depth()

[ Upstream commit c4ff1443 ]

This is useful helper to find out the depth of a connected router.
Convert the existing users to call this helper instead of open-coding.

No functional changes.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Qin Wan <qin.wan@hp.com>
Signed-off-by: Alexandru Gagniuc <alexandru.gagniuc@hp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>