[SCSI] aic79xx: fixup DT setting

this patch is just a cross-port of the fixup for aic7xxx DT settings.
As the same restrictions apply for aic79xx also (DT requires wide
transfers) the dt setting routine should be modified equivalently.
And an invalid period setting will be caught by ahd_find_syncrate()
anyway.

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