net: mvneta: mvneta_tx_done_gbe() cleanups

mvneta_tx_done_gbe() return value and third parameter are no more
used. This patch changes the function prototype and removes a useless
variable where the function is called.

Reviewed-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: David S. Miller <davem@davemloft.net>