Use BM/R series baud rate computation for FT230X.

Use BM/R series baud rate computation for FT230X.

The current code uses the AM series baud rate computation for FT230X,
and this results in incorrect divisor calculation for high baud rates.
Use the BM/R series logic instead.

This matches the corresponding code paths in the Linux kernel ftdi_sio
driver.  See ftdi_sio.c:1323 here:

http://lxr.free-electrons.com/source/drivers/usb/serial/ftdi_sio.c#L1323

Specifically attempts to set 2000000 on an FT230X fail currently due to
the if-statement at ftdi.c:1106.  The computed divisor (12) gets clamped
to 16, and thus the for-loop it's in doesn't converge on a usable divisor.

The if-statement at ftdi.c:1152 never fires, as the if-statement at
ftdi.c:1106 in the for-loop precludes a divisor of 12 (which is required
for *encoded_divisor to become 0x4001).