sfc: Introduce and use MCDI_DECLARE_BUF macro

MCDI_DECLARE_BUF declares a variable as an MCDI buffer of the
requested length, adding any necessary padding.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/mcdi.h b/drivers/net/ethernet/sfc/mcdi.h
index 3ba2e5b..c881118 100644
--- a/drivers/net/ethernet/sfc/mcdi.h
+++ b/drivers/net/ethernet/sfc/mcdi.h
@@ -85,6 +85,9 @@
 				   efx_qword_t *event);
 extern void efx_mcdi_sensor_event(struct efx_nic *efx, efx_qword_t *ev);
 
+#define MCDI_DECLARE_BUF(_name, _len)					\
+	u8 _name[ALIGN(_len, 4)]
+
 #define MCDI_PTR2(_buf, _ofst)						\
 	(((u8 *)_buf) + _ofst)
 #define MCDI_SET_DWORD2(_buf, _ofst, _value)				\