[ATALK]: Make CONFIG_DEV_APPLETALK a tristate.

Otherwise we allow building appletalk drivers in-kernel when
CONFIG_ATALK is modular.  That doesn't work because these drivers use
symbols such as "alloc_talkdev" which is exported from code built
by CONFIG_ATALK.

Noticed by Toralf Förster.

Signed-off-by: David S. Miller <davem@davemloft.net>