[SCSI] mvsas: a tag handler implementation

add a new tag handler to create slot num. When a slot num is busy, new
task can't hit this bit which was already used.  plumb in phy speeds.

Signed-off-by: Ke Wei <kewei@marvell.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>