isdn/capi: elliminate capincci_find() in non-middleware case

If Kernel CAPI is compiled without CONFIG_ISDN_CAPI_MIDDLEWARE,
the structure retrieved via capincci_find() is never actually
used, so don't compile that function in that case.

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