efi: Use NULL instead of 0 for pointer

Fix following sparse warnings:

drivers/firmware/efi/efivars.c:230:66: warning:
 Using plain integer as NULL pointer
drivers/firmware/efi/efi.c:236:27: warning:
 Using plain integer as NULL pointer

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index b25b36b..af20f17 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -233,7 +233,7 @@
 	{SAL_SYSTEM_TABLE_GUID, "SALsystab", &efi.sal_systab},
 	{SMBIOS_TABLE_GUID, "SMBIOS", &efi.smbios},
 	{UGA_IO_PROTOCOL_GUID, "UGA", &efi.uga},
-	{NULL_GUID, NULL, 0},
+	{NULL_GUID, NULL, NULL},
 };
 
 static __init int match_config_table(efi_guid_t *guid,