hw/char/pl011: better handling of FIFO flags on LCR reset

hw/char/pl011: better handling of FIFO flags on LCR reset

Current FIFO handling code does not reset RXFE/RXFF flags when guest
resets FIFO by writing to UARTLCR register, although internal FIFO state
is reset to 0 read count. Actual guest-visible flag update will happen
only on next data read or write attempt. As a result of that any guest
that expects RXFE flag to be set (and RXFF to be cleared) after resetting
FIFO will never see that happen.

Signed-off-by: Evgeny Iakovlev <eiakovlev@linux.microsoft.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230123162304.26254-5-eiakovlev@linux.microsoft.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>