serial: 8250_dw: only get the clock rate in one place

The clock rate is requested from a property called
"clock-frequency" in both dw8250_probe_of and
dw8250_probe_acpi. Moving the requests to dw8250_probe.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>