net: ethernet: dnet: Remove set but unused variable 'len'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/dnet.c: In function dnet_start_xmit
drivers/net/ethernet/dnet.c:511:15: warning: variable ‘len’ set but not used [-Wunused-but-set-variable]

commit 47964174 ("dnet: Dave DNET ethernet controller driver (updated)")
involved this unused variable, remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>