serial: imx: stop using USR2 in FIFO reading loop

The chip provides all the needed bits in the URXD0 register that we read
anyway for data, so get rid of reading USR2 and use only URXD0 bits
instead.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Link: https://lore.kernel.org/r/20230201142700.4346-7-sorganov@gmail.com


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