[SCSI] SAS: consolidate linkspeed definitions

At the moment we have two separate linkspeed enumerations covering
roughly the same values.  This patch consolidates on a single one enum
sas_linkspeed in scsi_transport_sas.h and uses it everywhere in the
aic94xx driver.  Eventually I'll get around to removing the duplicated
fields in asd_sas_phy and sas_phy ...

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