bnxt_en: selectively allocate context memories

Newer devices may have local context memory instead of relying on the
host for backing store. In these cases, HWRM_FUNC_BACKING_STORE_QCAPS
will return a zero entry size to indicate contexts for which the host
should not allocate backing store.

Selectively allocate context memory based on device capabilities and
only enable backing store for the appropriate contexts.

Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>