staging: et131x: Make rx_ring.fbr{0,1} share a common structure

Sharing a common structure by moving common structure items into
fbr_lookup.

TODO - Currently will not work if USE_FBR0 = 0 as FBR1 uses fbr[1]
which is removed in this case

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>