Add encode/decode for write response

The encode/decode functions for Write Response operations were created
to keep consistency with the rest of GATT API.
diff --git a/attrib/att.c b/attrib/att.c
index dff8597..3259fca 100644
--- a/attrib/att.c
+++ b/attrib/att.c
@@ -526,6 +526,27 @@
 	return len;
 }
 
+uint16_t enc_write_resp(uint8_t *pdu, int len)
+{
+	if (pdu == NULL)
+		return 0;
+
+	pdu[0] = ATT_OP_WRITE_RESP;
+
+	return sizeof(pdu[0]);
+}
+
+uint16_t dec_write_resp(const uint8_t *pdu, int len)
+{
+	if (pdu == NULL)
+		return 0;
+
+	if (pdu[0] != ATT_OP_WRITE_RESP)
+		return 0;
+
+	return len;
+}
+
 uint16_t enc_read_req(uint16_t handle, uint8_t *pdu, int len)
 {
 	const uint16_t min_len = sizeof(pdu[0]) + sizeof(handle);
diff --git a/attrib/att.h b/attrib/att.h
index 7e81dc4..7d9afeb 100644
--- a/attrib/att.h
+++ b/attrib/att.h
@@ -215,6 +215,8 @@
 							uint8_t *pdu, int len);
 uint16_t dec_write_req(const uint8_t *pdu, int len, uint16_t *handle,
 						uint8_t *value, int *vlen);
+uint16_t enc_write_resp(uint8_t *pdu, int len);
+uint16_t dec_write_resp(const uint8_t *pdu, int len);
 uint16_t enc_read_req(uint16_t handle, uint8_t *pdu, int len);
 uint16_t enc_read_blob_req(uint16_t handle, uint16_t offset, uint8_t *pdu,
 								int len);
diff --git a/src/attrib-server.c b/src/attrib-server.c
index 72f5b17..85b39a8 100644
--- a/src/attrib-server.c
+++ b/src/attrib-server.c
@@ -601,9 +601,7 @@
 	memcpy(&uuid, &a->uuid, sizeof(uuid_t));
 	attrib_db_update(handle, &uuid, value, vlen);
 
-	pdu[0] = ATT_OP_WRITE_RESP;
-
-	return sizeof(pdu[0]);
+	return enc_write_resp(pdu, len);
 }
 
 static uint16_t mtu_exchange(struct gatt_channel *channel, uint16_t mtu,