driver core: let dev_set_drvdata return int instead of void as it can fail

Before commit

	b4028437 (Driver core: move dev_get/set_drvdata to drivers/base/dd.c)

calling dev_set_drvdata with dev=NULL was an unchecked error. After some
discussion about what to return in this case removing the check (and so
producing a null pointer exception) seems fine.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>