libata: Check PCI sub-class code before disabling AHCI

This patch adds functionality to check the PCI sub-class code of an
AHCI capable device before disabling AHCI.  It fixes a bug where an
ICH7 sata controller is being setup by the BIOS as sub-class 1 (ide)
and the AHCI control registers weren't being initialized, thus causing
an IO error in piix_disable_ahci().

Signed-off-by: Gregory Felix <greg.felix@gmail.com>