sata_nv: enable hotplug interrupt and fix some readl/readw mismatches

We already have code that handles hotplug interrupt indications in ADMA
mode, this turns on the control flag that actually enables these interrupts.
Also fixes some cases in the same functions where a 16-bit register was read
using a readl instead of a readw.

Signed-off-by: Robert Hancock <hancockr@shaw.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>