ARM: rpc: check device_register return code in ecard_probe

device_register is marked __must_check, so we better propagate the error
value by returning it from ecard_probe.

Without this patch, building rpc_defconfig results in:

arch/arm/mach-rpc/ecard.c: In function 'ecard_probe':
arch/arm/mach-rpc/ecard.c:963:17: warning: ignoring return value of 'device_register', declared with attribute warn_unused_result [-Wunused-result]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mach-rpc/ecard.c

@@ -960,7 +960,9 @@ static int __init ecard_probe(int slot, unsigned irq, card_type_t type)
 	*ecp = ec;
 	slot_to_expcard[slot] = ec;
 
-	device_register(&ec->dev);
+	rc = device_register(&ec->dev);
+	if (rc)
+		goto nodev;
 
 	return 0;