staging: et131x: Align ring_index and fbr[] indicies

A ring_index value of 0 is being used to reference the free buffer ring
fbr[1] and ring_index=1 is used to reference fbr[0]. Avoid this
confusing referencing by swapping the indicies so that ring_index=0
refers to fbr[0] and vice versa.

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