Return the error from ahci_setup_interrupt in ahci_attach

Previously ahci_attach returned a hard coded ENXIO instead of the value
from ahci_setup_interrupt. This is effectively a NOOP change as currently
ahci_setup_interrupt only ever returns 0 or ENXIO, so just there to protect
against any future changes to that.

Differential Revision: D838
MFC after: 2 weeks
Sponsored by: Multiplay