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

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>

drivers/thunderbolt/Kconfig

@@ -16,9 +16,10 @@ menuconfig USB4
 	  To compile this driver a module, choose M here. The module will be
 	  called thunderbolt.
 
+if USB4
+
 config USB4_DEBUGFS_WRITE
 	bool "Enable write by debugfs to configuration spaces (DANGEROUS)"
-	depends on USB4
 	help
 	  Enables writing to device configuration registers through
 	  debugfs interface.
@@ -28,5 +29,6 @@ config USB4_DEBUGFS_WRITE
 
 config USB4_KUNIT_TEST
 	bool "KUnit tests"
-	depends on USB4
 	depends on KUNIT=y
+
+endif # USB4