thunderbolt: Use "if USB4" instead of "depends on" in Kconfig

commit 8eabfca5 upstream.

This groups the USB4 options more nicely, and also does not require that
every config option lists explicit depends on USB4.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>