serial: 8250_dw: Avoid pslverr on reading empty receiver fifo

With PSLVERR_RESP_EN parameter set to 1, the device generates an error
response when an attempt to read an empty RBR with FIFO enabled.

This happens when LCR writes are ignored when UART is busy.
dw8250_check_lcr() in retries to update LCR, invokes dw8250_force_idle()
to clear and reset FIFO and eventually reads UART_RX causing the error.

Avoid this by not reading RBR/UART_RX when no data is available.

Reviewed-by: Andy Shevchenko <[email protected]>
Signed-off-by: VAMSHI GAJJELA <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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