serial: 8250_pci: Correct uartclk for xr17v35x expansion chips

serial: 8250_pci: Correct uartclk for xr17v35x expansion chips

commit 899f0c1c7dbcc487fdc8756a49ff70b1d5d75f89 upstream.

The internal clock of the master chip, which is usually 125MHz, is only half
(62.5MHz) for the slave chips. So we have to adjust the uartclk for all the
slave ports. Therefor we add a new function to determine if a slave chip is
present and update pci_xr17v35x_setup accordingly.

Signed-off-by: Soeren Grunewald <soeren.grunewald@desy.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>