imx6-mmdc: fix automatic power down enable in write level calibration
Bit 0 of the MAPSR register controls auto power down.
Explicitly clear this bit instead of reserved bit when
exiting from mmdc_do_write_level_calibration().
Signed-off-by: Eric Nelson <eric@nelint.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>