serial: core: remove baud_rates when serial console setup

Currently, when tring to set up a serial console with a higher
baud rate, it would fallback to 921600.

Tested-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>