[libata sata_mv] handle lack of hardware nIEN support

Handle errata (it was unintentional on this h/w, whereas its intentional
on others) whereby the nIEN bit in Device Control is ignored, leading to
a situation where a hardware interrupt completes the qc before the
polling code has a chance to.

This will get fixed The Right Way(tm) once Albert Lee's irq-pio
branch is merged, as the more natural PIO method on this hardware is
interrupt-driven.