sfc: Fix sign of efx_mcdi_poll_reboot() error in efx_mcdi_poll()

efx_mcdi_poll() uses positive error numbers, matching the MCDI
protocol.  It must negate the result of efx_mcdi_poll_reboot() which
returns the usual negative error numbers.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/sfc/mcdi.c

@@ -127,7 +127,7 @@ static int efx_mcdi_poll(struct efx_nic *efx)
 	efx_dword_t reg;
 
 	/* Check for a reboot atomically with respect to efx_mcdi_copyout() */
-	rc = efx_mcdi_poll_reboot(efx);
+	rc = -efx_mcdi_poll_reboot(efx);
 	if (rc)
 		goto out;