serial: sh-sci: Improve comments for DMA timeout calculation

#ifdef CONFIG_SERIAL_SH_SCI_DMA

	/*

	 * Calculate delay for 2 DMA buffers (4 FIFO).

	 * See serial_core.c::uart_update_timeout(). With 10

	 * bits (CS8), 250Hz, 115200 baud and 64 bytes FIFO, the above function

	 * calculates 1 jiffie for the data plus 5 jiffies for the "slop(e)."

	 * Then below we calculate 5 jiffies (20ms) for 2 DMA buffers (4 FIFO

	 * sizes), but when performing a faster transfer, value obtained by

	 * this formula is may not enough. Therefore, if value is smaller than

	 * 20msec, this sets 20msec as timeout of DMA.

	 * See serial_core.c::uart_update_timeout().

	 * With 10 bits (CS8), 250Hz, 115200 baud and 64 bytes FIFO, the above

	 * function calculates 1 jiffie for the data plus 5 jiffies for the

	 * "slop(e)." Then below we calculate 5 jiffies (20ms) for 2 DMA

	 * buffers (4 FIFO sizes), but when performing a faster transfer, the

	 * value obtained by this formula is too small. Therefore, if the value

	 * is smaller than 20ms, use 20ms as the timeout value for DMA.

	 */

	if (s->chan_rx) {

		unsigned int bits;