net: thunderbolt: Move into own directory

We will be adding tracepoints to the driver so instead of littering the
main network driver directory, move the driver into its own directory.
While there, rename the module to thunderbolt_net (with underscore) to
match with the thunderbolt_dma_test convention.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>