ns16550 RX port clear on configure is causing SLVERR on DesignWare instance

[texas-chris]

The current ns16550 attempts to clear the RX port in the uart_ns16550_configure function. On Synopsys DesignWare IP that has the PSLVERR_RESP_EN parameter enabled, if the RX FIFO is empty and a read is performed a SLVERR will be generated. This assumes no data arrives before the register read/write timeout occurs. On ARM this generates a 'Data Abort' and halts the OS from booting.

Is there any reason this isn't wrapped with a check LSR bit RXRDY? This would eliminate the potential SLVERR and also drain the entire RXFIFO, instead of just one byte.

Thoughts?

Potential fix:

/* clear the port */
while((ns16550_inbyte(dev_cfg, LSR(dev)) & LSR_RXRDY) != 0) {
	ns16550_inbyte(dev_cfg, RDR(dev));
}