thunderbolt: Drop duplicated get_switch_at_route()

[ Upstream commit 8f965efd215a09c20b0b5e5bb4e20009a954472e ]

tb_switch_find_by_route() does the same already so use it instead and
remove duplicated get_switch_at_route().

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>