[SCSI] st: fix kernel BUG at include/linux/scatterlist.h:59!

This is caused by a missing scatterlist initialisation (it only shows
up when sg list handling debugging is turned on).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Kai Makisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>