be2iscsi: Fix bogus WARN_ON length check

drivers/scsi/be2iscsi/be_main.c: In function 'be_sgl_create_contiguous':
drivers/scsi/be2iscsi/be_main.c:3187:18: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
  WARN_ON(!length > 0);

gcc version 5.2.1

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Cc: Jayamohan Kallickal <jayamohan.kallickal@avagotech.com>
Cc: Minh Tran <minh.tran@avagotech.com>
Cc: John Soni Jose <sony.john-n@avagotech.com>
Cc: "James E.J. Bottomley" <JBottomley@odin.com>
Reported-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Manoj Kumar <manoj@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>