ahci: make ahci_asus_m2a_vm_32bit_only() quirk more generic
It turns out ASUS M2A-VM isn't the only one with the 32bit DMA
problem. Make ahci_asus_m2a_vm_32bit_only() more generic using the
new dmi_get_date() and rename it to ahci_sb600_32bit_only(). Cut off
date is now pointed to by dmi_system_id->driver_data in "yyyymmdd"
format and it's now also allowed to be omitted.