serial: sh-sci: Simplify baud rate calculation algorithms

Rewrite the baud rate register value calculations in easier to read
forms. The computed value isn't modified.

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>