powerpc/nvram_64: Check nvram_error_log_index in nvram_clear_error_log()

nvram_clear_error_log() calls ppc_md.nvram_write() even when
nvram_error_log_index is -1 (invalid). The nvram_write() function does
not check for a negative offset.

Check nvram_error_log_index as the other nvram log functions do.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/nvram_64.c

@@ -681,6 +681,9 @@ int nvram_clear_error_log(void)
 	int clear_word = ERR_FLAG_ALREADY_LOGGED;
 	int rc;
 
+	if (nvram_error_log_index == -1)
+		return -1;
+
 	tmp_index = nvram_error_log_index;
 	
 	rc = ppc_md.nvram_write((char *)&clear_word, sizeof(int), &tmp_index);