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: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/sfc/mcdi.c b/drivers/net/sfc/mcdi.c
index 9f035b9..f66b3da 100644
--- a/drivers/net/sfc/mcdi.c
+++ b/drivers/net/sfc/mcdi.c
@@ -127,7 +127,7 @@
 	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;