serial: imx: only set receiver level if it is zero

With commit a81dbd04 ("serial: imx: set receiver level before
starting uart") we set the receiver level to its default value. This
caused a regression when using SDMA, where the receiver level is 9
instead of 8 (default). This change will first check if the receiver
level is zero and only then set it to the default. This still avoids the
interrupt storm when the receiver level is zero.

Fixes: a81dbd04

 ("serial: imx: set receiver level before starting uart")
Cc: stable <stable@kernel.org>
Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Link: https://lore.kernel.org/r/20240703112543.148304-1-eichest@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>