Since kmalloc_node() is called with __GFP_ZERO flag there is no need to
explicitly zero hwgroup fields.

While at it:

* Use 'hwif' instead of 'hwif->drives[0].hwif' for kmalloc_node() call.

* Fix whitespace damage.
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>