serial: sifive: Remove duplicate `clkin_rate' setting

The `clkin_rate' member of `struct sifive_serial_port' now duplicates
`uartclk' from nested `struct uart_port', so use `uartclk' throughout
and remove `clkin_rate'.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204291656150.9383@angie.orcam.me.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>