serial: 8250: Correct the clock for OxSemi PCIe devices

Oxford Semiconductor PCIe (Tornado) serial port devices are driven by a
fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.

In the enhanced (650) mode, which we select in `autoconfig_has_efr' by
setting the ECB bit in the EFR register, and in the absence of clock
reconfiguration, which we currently don't do, the clock rate is divided
only by the oversampling rate of 16 as it is supplied to the baud rate
generator, yielding the baud base of 3906250.  This comes from the reset
values of the TCR and MCR[7] registers which are both zero[1][2][3][4],
choosing the oversampling rate of 16 and the normal (divide by 1) baud
rate generator prescaler respectively.  This is the rate that is divided
by the value held in the divisor latch to determine the baud rate used.

Replace the incorrect baud base of 4000000 with the right value of
3906250 then.

References:

[1] "OXPCIe200 PCI Express Multi-Port Bridge", Oxford Semiconductor,
    Inc., DS-0045, 10 Nov 2008, Section "Reset Configuration", p. 72

[2] "OXPCIe952 PCI Express Bridge to Dual Serial & Parallel Port",
    Oxford Semiconductor, Inc., DS-0046, Mar 06 08, Section "Reset
    Configuration", p. 27

[3] "OXPCIe954 PCI Express Bridge to Quad Serial Port", Oxford
    Semiconductor, Inc., DS-0047, Feb 08, Section "Reset Configuration",
    p. 28

[4] "OXPCIe958 PCI Express Bridge to Octal Serial Port", Oxford
    Semiconductor, Inc., DS-0048, Feb 08, Section "Reset Configuration",
    p. 28

Fixes: 7106b4e3

 ("8250: Oxford Semiconductor Devices")
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2106100203510.5469@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>