serial: 8250: Actually allow UPF_MAGIC_MULTIPLIER baud rates

serial: 8250: Actually allow UPF_MAGIC_MULTIPLIER baud rates

Support for magic baud rate divisors of 32770 and 32769 used with SMSC
Super I/O chips for extra baud rates of 230400 and 460800 respectively
where base rate is 115200[1] has been added around Linux 2.5.64, which
predates our repo history, but the origin could be identified as commit
2a717aad772f ("Merge with Linux 2.5.64.") with the old MIPS/Linux repo
also at: <git://git.kernel.org/pub/scm/linux/kernel/git/ralf/linux.git>.

Code that is now in `serial8250_do_get_divisor' was added back then to
`serial8250_get_divisor', but that code would only ever trigger if one
of the higher baud rates was actually requested, and that cannot ever
happen, because the earlier call to `serial8250_get_baud_rate' never
returns them.  This is because it calls `uart_get_baud_rate' with the
maximum requested being the base rate, that is clk/16 or 115200 for SMSC
chips at their nominal clock rate.

Fix it then and allow UPF_MAGIC_MULTIPLIER baud rates to be selected, by
requesting the maximum baud rate of clk/4 rather than clk/16 if the flag
has been set.  Also correct the minimum baud rate, observing that these
ports only support actual (non-magic) divisors of up to 32767 only.

References:

[1] "FDC37M81x, PC98/99 Compliant Enhanced Super I/O Controller with
    Keyboard/Mouse Wake-Up", Standard Microsystems Corporation, Rev.
    03/27/2000, Table 31 - "Baud Rates", p. 77

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2105190412280.29169@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>