[SCSI] aci94xx: implement link rate setting

This patch implements the ability to set the minimum and maximum
linkrates for both libsas (for expanders) and aic94xx (for the host
phys).  It also tidies up the setting of the hardware min and max to
make sure they're updated when the expander emits a change broadcast.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>