serial: sh-sci: Update calculation of timeout for DMA

The current calculation method in the case of 9600bps, rounding error occurs
has become setting that occur timeout faster than the required time. When we
use 9600bps, 32byte buffer, 10 bit (CS8) and 100 HZ, it becomes 3 jiffies
(30msec). In fact it is necessary 33msec. This updates to the calculation
that are not actually less than the value set by the rounding error.
Also, this is nothing will be calculated value when there is no load. If there
are a lot of case load, overrun error will occur immediately.
This is by the buffer size to be calculated twice the DMA buffer, and add the
change of setting a sufficient time-out value.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>