hvc_vio: Do not override preferred console set by kernel parameter

The original version of this was done by Bastian Blank, who wrote:

> The problem is the following:
> - Architecture specific code sets preferred console to something bogus.
> - Command line handling tries to set preferred console but is overruled
>   by the old setting.
>
> The udbg0 console is a boot console and independant.

References: http://bugs.debian.org/492703


Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>