edd: fix possible memory leak in edd_init() error path

The error may happen at any iteration of the for loop, this patch properly
unregisters already registed edd_devices in error path.

[akpm@linux-foundation.org: remove unneeded NULL test]
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>