add mrsas_shutdown method

It should be safer to flush controller and disk caches on the shutdown.
And to gracefully shut down the controller as well.
It seems that the Linux driver has been doing that for a long time.

Discussed with: scottl
Reviewed by: imp, Sumit Saxena <sumit.saxena@broadcom.com>

		(both earlier version)

MFC after: 3 weeks
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D19817