USB: serial: ch341: reimplement line-speed handling

USB: serial: ch341: reimplement line-speed handling

The current ch341 divisor algorithm was known to give inaccurate results
for certain higher line speeds. Jonathan Olds <jontio@i4free.co.nz>
investigated this, determined the basic equations used to derive the
divisors and confirmed them experimentally [1].

The equations Jonathan used could be generalised further to:

	baudrate = 48000000 / (2^(12 - 3 * ps - fact) * div), where

		0 <= ps <= 3,
		0 <= fact <= 1,
		2 <= div <= 256 if fact = 0, or
		9 <= div <= 256 if fact = 1

which will also give better results for lower rates.

Notably the error is reduced for the following standard rates:

	1152000	(4.0% instead of 15% error)
	 921600	(0.16% instead of -7.5% error)
	 576000	(-0.80% instead of -5.6% error)
	    200	(0.16% instead of -0.69% error)
	    134	(-0.05% instead of -0.63% error)
	    110	(0.03% instead of -0.44% error)

but also for many non-standard ones.

The current algorithm also suffered from rounding issues (e.g.
requesting 2950000 Bd resulted in a rate of 2 MBd instead of 3 MBd and
thus a -32% instead of 1.7% error).

The new algorithm was inspired by the current vendor driver even if that
one only handles two higher rates that require fact=1 by hard coding the
corresponding divisors [2].

Michael Dreher <michael@5dot1.de> also did a similar generalisation of
Jonathan's work and has published his results with a very good summary
that provides further insights into how this device works [3].

[1] https://lkml.kernel.org/r/000001d51f34$bad6afd0$30840f70$@co.nz
[2] http://www.wch.cn/download/CH341SER_LINUX_ZIP.html
[3] https://github.com/nospam2000/ch341-baudrate-calculation

Reported-by: Jonathan Olds <jontio@i4free.co.nz>
Tested-by: Jonathan Olds <jontio@i4free.co.nz>
Cc: Michael Dreher <michael@5dot1.de>
Signed-off-by: Johan Hovold <johan@kernel.org>