[SCSI] st: move st_request initialization to st_allocate_request form st_do_scsi

This moves st_request initialization code to st_allocate_request()
form st_do_scsi(). This is a preparation for making
st_allocate_request() usable for everyone, not only st_do_scsi().

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