sparc: Fix cleanup crash in bbc_envctrl_cleanup()

If kthread_run() fails or never gets to run we'll have NULL
or a pointer encoded error in kenvctrld_task, rather than
a legitimate task pointer.

So this makes bbc_envctrl_cleanup() crash as it passed this
bogus pointer into kthread_stop().

Reported-by: BERTRAND Joël <joel.bertrand@systella.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>