i2c: mxs: change error printing to debug for mxs_i2c_pio_wait_xfer_end

Instead of printing errors after mxs_i2c_pio_wait_xfer_end returns with
an error code just print a debug message.

NAKs and timeouts can occur in this situation normally, so do not treat
them as errors.

Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.at>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>