ipa: rename `struct ipa_link' by `struct ipa_client_link'

And IPA_LINK_STATE_* by IPA_CLIENT_LINK_STATE_* to prepare
the addition of the ipa_server_link abstraction.
diff --git a/src/input/ipaccess.c b/src/input/ipaccess.c
index 19e5a53..60e7109 100644
--- a/src/input/ipaccess.c
+++ b/src/input/ipaccess.c
@@ -428,7 +428,7 @@
 	return 0;
 }
 
-static int ipaccess_bts_cb(struct ipa_link *link, struct msgb *msg)
+static int ipaccess_bts_cb(struct ipa_client_link *link, struct msgb *msg)
 {
 	struct ipaccess_head *hh = (struct ipaccess_head *) msg->data;
 	struct e1inp_ts *e1i_ts = NULL;
@@ -519,7 +519,7 @@
 		}
 		break;
 	case E1INP_LINE_R_BTS: {
-		struct ipa_link *link, *rsl_link;
+		struct ipa_client_link *link, *rsl_link;
 
 		LOGP(DINP, LOGL_NOTICE, "enabling ipaccess BTS mode\n");