Reduce the diff to the arm_intrng project branch by having the read/write
macros take the softc they are accessing.
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
Reduce the diff between head and arm_intrng with the bcm2835 interrupt
controller.
style (9) nits

Use return (val); instead of return val;

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