Fix enum warning in keyserv

This fixes a clang 19 warning:

usr.sbin/keyserv/crypt_server.c:237:53: error: comparison of different enumeration types ('des_mode' (aka 'enum des_mode') and 'enum desmode') [-Werror,-Wenum-compare]
  237 |         if (_my_crypt != &_arcfour_crypt && argp->des_mode == CBC) {
      |                                             ~~~~~~~~~~~~~~ ^  ~~~

The type of argp->des_mode (aka desargs::des_mode) is `enum
des_mode` from /usr/include/rpcsvc/crypt.h, not enum desmode from
/usr/include/rpc/des.h (which is used in struct desparams).

Luckily the integer values of enum desmode's CBC and ECB are
identical to those of enum des_mode's CBC_DES and ECB_DES, so
replace both values.

MFC after: 3 days