serial: 8250_early: Setup divider when uartclk is passed

device->baud is always non zero value because it is checked already in
early_serial8250_setup() before init_port is called.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>