[SCSI] libsas: SMP request handler shouldn't crash when rphy is NULL

sas_smp_handler crashes when smp utils are used with an aic94xx host
because certain devices (the sas_host itself, specifically) lack rphy
structures.  No rphy means no SMP target support, but we shouldn't crash
here.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>