[SCSI] mptsas: support basic hotplug

Adds hotplug support for SAS end devices.  Unfortunately the fusion
firmware doesn't generate similar events for expanders addition/removal
so we can't support them yet.  Eric has an idea about a clever scheme to
find out about expander changes so that'll be added later on.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>