usb: serial: ftdi_sio.c: For the FT232H FTDI_SIO_SET_BAUDRATE_REQUEST, index needs to be shifted too

the recent addition of the FT232H showed that baudrate was set wrong. See
gmane.linux.usb.general: "[ftdi_sio] FT232H support".  With the old code,
the MSB of the 4 encoded fractional divider bits and more important the
clock predivider bits got lost. Adding the FT232H to the code patch were
these bits are shifted solves the problem. I verified baud rates with a
scope now.

I suspect, that the BM device probably needs these bits shifted too. But
there is no predivider bit, so this is not obvious, and a missing MSB of the
encoded fractional divider only shifts the resulting baudrate minimal.
The AM has only 3 bits of encoded fractional divider, so it is not impacted.

I have no BM device to test, so I only added a comment and left the code for
the BM untouched.
Signed-off-by: NUwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>