Fix breakage introduced in r238824 - correctly calculate the descriptor

Fix breakage introduced in r238824 - correctly calculate the descriptor
wrapping.

The previous code was only wrapping descriptor "block" boundaries rather
than individual descriptors.  It sounds equivalent but it isn't.

r238824 changed the descriptor allocation to enforce that an individual
descriptor doesn't wrap a 4KiB boundary rather than the whole block
of descriptors.  Eg, for TX descriptors, they're allocated in blocks
of 10 descriptors for each ath_buf (for scatter/gather DMA.)