earlycon: Remove hardcoded port->uartclk initialization in of_setup_earlycon

There is no reason to initialize uartclk to BASE_BAUD * 16 for DT based
systems.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>