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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Sandor Bodo-Merle <sbodomerle@gmail.com>
Cc: Shane Huang <shane.huang@amd.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>