ccr: Retire ccr_softc member in struct adapter.

Prior to Conrad's changes to replace session integer IDs with a
pointer to the driver-specific state in commit 1b0909d51a8aa, the
driver had to find the softc pointer from the adapter before it could
locate the ccr_session structure for a completed request. Since
Conrad's changes, the ccr_session pointer can now be obtained directly
from the crp. Add a backpoint from ccr_session back to ccr_softc and
use this in place of the ccr_softc member in cxgbe's struct adapter.

Sponsored by: Chelsio Communications