[SCSI] scsi_transport_sas: add destructor for bsg

There's currently no destructor for the bsg components.  If you insert
and remove the module, you see the bsg devices building up and up.  This
patch adds the destructor in the correct place in the transport class so
that the bsg and request queue are removed just before the device
destruction.

Acked-by: FUJITA Tomonori <tomof@acm.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>