cpu: let spi_init_slave() stubs return error codes

cpu/lpc2387/periph/spi.c

@@ -141,7 +141,7 @@ int spi_init_slave(spi_t dev, spi_conf_t conf, char (*cb)(char))
     (void)cb;
     printf("%s:%s(): stub\n", RIOT_FILE_RELATIVE, __func__);
     /* TODO */
-    return 0;
+    return -1;
 }
 
 void spi_transmission_begin(spi_t dev, char reset_val)

cpu/samd21/periph/spi.c

@@ -200,7 +200,7 @@ int spi_init_master(spi_t dev, spi_conf_t conf, spi_speed_t speed)
 int spi_init_slave(spi_t dev, spi_conf_t conf, char (*cb)(char))
 {
     /* TODO */
-    return 0;
+    return -1;
 }
 
 void spi_transmission_begin(spi_t dev, char reset_val)

cpu/saml21/periph/spi.c

@@ -173,7 +173,7 @@ int spi_init_master(spi_t dev, spi_conf_t conf, spi_speed_t speed)
 int spi_init_slave(spi_t dev, spi_conf_t conf, char (*cb)(char))
 {
     /* TODO */
-    return 0;
+    return -1;
 }
 
 void spi_transmission_begin(spi_t dev, char reset_val)