staging: et131x: Fix issues when USE_FBR0 is not defined

* Following on from making rx_ring.fbr use a common structure - reversed
the fbr[] array indicies so that index 1 = FBR0 and index 0 = FBR1,
which allows USE_FBR0 define to work.

* Also fixed up minor issues where indexes into the array were out of
bounds in some places.

* Removed rx_ring.fbr common stuct TODO item from README
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/et131x/README

@@ -8,7 +8,6 @@ Note, the powermanagement options were removed from the vendor provided
 driver as they did not build properly at the time.
 
 TODO:
-	- rx_ring.fbr{0, 1} can probably share a common structure
 	- Use of kmem_cache seems a bit unusual
 	- Use dma_alloc_... in place of pci_alloc_...
 	- It's too late stopping the tx queue when there is no room for the current packet. The condition should be detected for the next packet.