isdn/capi: Make Middleware depend on CAPI2.0

The Kconfig symbol ISDN_CAPI_MIDDLEWARE is only used in capi.c. Setting
it without setting ISDN_CAPI_CAPI20 is therefor useless. Make it depend
on ISDN_CAPI_CAPI20 and put its entry after ISDN_CAPI_CAPI20's entry.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>