mfd: Remove unneeded and dangerous clearing of clientdata

Unlike real i2c-devices which get detached from the driver, dummy-devices
get truly unregistered. So, there has never been a need to clear the
clientdata because the device will go away anyhow. For the occasions fixed
here, clearing clientdata was even dangerous as the structure was freed
already.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>