staging: et131x: Remove unused rx_ring.recv_buffer_pool

rx_ring.recv_buffer_pool is unused, even in the original driver code.
Remove from stuct, and also remove some comments regarding it.

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