serial: imx: work-around for hardware RX flood

Check if hardware Rx flood is in progress, and issue soft reset to UART to
stop the flood.

A way to reproduce the flood (checked on iMX6SX) is: open iMX UART at 9600
8N1, and from external source send 0xf0 char at 115200 8N1. In about 90% of
cases this starts a flood of "receiving" of 0xff characters by the iMX UART
that is terminated by any activity on RxD line, or could be stopped by
issuing soft reset to the UART (just stop/start of RX does not help). Note
that in essence what we did here is sending isolated start bit about 2.4
times shorter than it is to be if issued on the UART configured baud rate.

There was earlier attempt to fix similar issue in: 'commit
b38cb7d25711 ("serial: imx: Disable new features of autobaud detection")',
but apparently it only gets harder to reproduce the issue after that
commit.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Link: https://lore.kernel.org/r/20230201142700.4346-3-sorganov@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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