printk: Check valid console index for preferred console

Let's check for valid console index values for preferred console to avoid
bogus console index numbers from kernel command line.

Let's also return an error for negative index numbers for the preferred
console. Unlike for device drivers, a negative index is not valid for the
preferred console.

Let's also constify idx while at it.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20231012064300.50221-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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