[SCSI] megaraid_sas: check return value for megasas_get_pd_list()

When megasas_get_pd_list() fails we cannot detect any drives,
so we should be checking the return value accordingly.

[jejb: checkpatch fix]
Signed-off-by: Hannes Reinecke <hare@suse.de>
Acked-by: Kashyap Desai <kashyap.desai@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>