xen/hvc: allow xenboot console to be used again

commit a9fbf4d591da6cd1d3eaab826c7c15f77fc8f6a3 upstream.

Commit d0380e6c3c0f6edb986d8798a23acfaf33d5df23 (early_printk:
consolidate random copies of identical code) added in 3.10 introduced
a check for con->index == -1 in early_console_register().

Initialize index to -1 for the xenboot console so earlyprintk=xen
works again.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
index 682210d..4fc32c8 100644
--- a/drivers/tty/hvc/hvc_xen.c
+++ b/drivers/tty/hvc/hvc_xen.c
@@ -636,6 +636,7 @@
 	.name		= "xenboot",
 	.write		= xenboot_write_console,
 	.flags		= CON_PRINTBUFFER | CON_BOOT | CON_ANYTIME,
+	.index		= -1,
 };
 #endif	/* CONFIG_EARLY_PRINTK */