efi: Update some comments related to smbios tables

Clarify the operation of this code with some additional comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

include/efi_loader.h

@@ -214,6 +214,13 @@ efi_status_t efi_net_register(void);
 /* Called by bootefi to make the watchdog available */
 efi_status_t efi_watchdog_register(void);
 /* Called by bootefi to make SMBIOS tables available */
+/**
+ * efi_smbios_register() - write out SMBIOS tables
+ *
+ * Called by bootefi to make SMBIOS tables available
+ *
+ * @return 0 if OK, -ENOMEM if no memory is available for the tables
+ */
 efi_status_t efi_smbios_register(void);
 
 struct efi_simple_file_system_protocol *