Fix ahci(4) driver attach to controller with 32 ports.

Incorrect sign expansion in variables that supposed to be a bit fields
caused infinite loop. Fixing this allows system properly detect maximal
possible 32 devices configured on AHCI HBA of BHyVe. That case did not
happen in a wild before due to lack of hardware AHCI HBAs with 32 ports.

Approved by: re (gjb@)
MFC after: 1 week