mtd/maps/pismo: remove dangling pointer and a leak

While looking for drivers which forgot to clear i2c_clientdata before freeing
the data structure it points to, I found that the pismo driver even has a leak
on the probe error path.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>