x86: early_printk: Protect against using the same device twice

If you use the kernel argument:

  earlyprintk=serial,ttyS0,115200

This will cause a recursive hang printing the same line
again and again:

 BIOS-e820: 000000003fff3000 - 0000000040000000 (ACPI data)
 BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
 BIOS-e820: 00000000fec00000 - 0000000100000000 (reserved)
bootconsole [earlyser0] enabled
Linux version 2.6.31-07863-gb64ada6 (mingo@sirius) (gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) ) #16789 SMP Wed Sep 23 21:09:43 CEST 2009
Linux version 2.6.31-07863-gb64ada6 (mingo@sirius) (gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) ) #16789 SMP Wed Sep 23 21:09:43 CEST 2009
Linux version 2.6.31-07863-gb64ada6 (mingo@sirius) (gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) ) #16789 SMP Wed Sep 23 21:09:43 CEST 2009
Linux version 2.6.31-07863-gb64ada6 (mingo@sirius) (gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) ) #16789 SMP Wed Sep 23 21:09:43 CEST 2009
Linux version 2.6.31-07863-gb64ada6 (mingo@sirius) (gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) ) #16789 SMP Wed Sep 23 21:09:43 CEST 2009

Instead warn the end user that they specified the device
a second time, and ignore that second console.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Greg KH <gregkh@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <4ABAAB89.1080407@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>