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>