Isolate CAIF transport drivers into their own menu.

This cleans up the main Network device support menu,
makes it easier to find the CAIF drivers, and makes it
easier to enable/disable them as a group.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>