[SCSI] handle scsi_add_host failure for aic79xx and fix compiler warning

Add scsi_add_host() failure handling for aic79xx
Also silence a compiler warning :
 drivers/scsi/aic7xxx/aic79xx_osm.c: In function `ahd_linux_register_host':
 drivers/scsi/aic7xxx/aic79xx_osm.c:1099: warning: ignoring return value of `scsi_add_host', declared with attribute warn_unused_result

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>