handle the status ring entries as 64bit words instead of a struct.

handle the status ring entries as 64bit words instead of a struct.

the status ring entries are 8 bytes/64bit, and depending on the
type of entry it has fields all over the place. this loads the
descriptor with a single 64bit read, and then shifts and masks the
bits out of it depending on the type of descriptor. this looks
cleaner for the tx completions in particular.