staging: et131x: Tidy up rx/tx dropped & bytes stats

Remove some fairly useless comments regarding rx/tx _bytes and
_dropped, and use rcvd_pkts_dropped stat value to provide rx_dropped.

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