ARM: mmp: Fix failure to remove sram device

Make sure in .probe() to set driver data before the function is left to
make it possible in .remove() to undo the actions done.

This fixes a potential memory leak and stops returning an error code in
.remove() that is ignored by the driver core anyhow.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>