serial: imx: refine local variables in rxint()

The 'rx' is chip register, similar to 'usr2', so let it be of 'u32' type as
well.

Move 'flg' to be FIFO read loop local as it's not used outside.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Link: https://lore.kernel.org/r/20230201142700.4346-8-sorganov@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>