bnxt_re: Implement the shutdown hook of the L2-RoCE driver interface

When host is shutting down, it invokes the shutdown hook of the
L2 driver where it would attempt to free the MSI-X vectors, but would fail
because some vectors are held by the RoCE driver.
Implement the new hook in the L2 -> RoCE interface which will be invoked so that
the RoCE driver can unregister the device and free up the MSI-X vectors it had
claimed so that L2 can proceed with it's shutdown without failure.

Signed-off-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>