USB: serial: ch341: add min and max line-speed macros

The line-speed algorithm clamps the requested value to the supported
range instead of bailing out on unsupported values.

Provide min and max macros and indicate how they are derived instead of
hardcoding the limits.

Note that the algorithm depends on the minimum rate (45.78 bps)
being rounded up (and the maximum rate being rounded down) to avoid
special casing.
Suggested-by: Michael Hanselmann <public@hansmi.ch>
Link: https://lore.kernel.org/r/20200630095756.GZ3334@localhostSigned-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ch341.c

@@ -156,6 +156,10 @@ static const speed_t ch341_min_rates[] = {
 	CH341_MIN_RATE(3),
 };
 
+/* Supported range is 46 to 3000000 bps. */
+#define CH341_MIN_BPS	DIV_ROUND_UP(CH341_CLKRATE, CH341_CLK_DIV(0, 0) * 256)
+#define CH341_MAX_BPS	(CH341_CLKRATE / (CH341_CLK_DIV(3, 0) * 2))
+
 /*
  * The device line speed is given by the following equation:
  *
@@ -177,7 +181,7 @@ static int ch341_get_divisor(struct ch341_private *priv)
 	 * Clamp to supported range, this makes the (ps < 0) and (div < 2)
 	 * sanity checks below redundant.
 	 */
-	speed = clamp(speed, 46U, 3000000U);
+	speed = clamp_val(speed, CH341_MIN_BPS, CH341_MAX_BPS);
 
 	/*
 	 * Start with highest possible base clock (fact = 1) that will give a