serial: sh-sci: Compute overrun_bit without using baud rate algo

The overrun bit index is a property of the hardware. It's currently
computed based on a different and unrelated hardware property, the baud
rate calculation algorithm. Compute it using hardware identification
information only.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>