staging: et131x: remove spinlock adapter->lock

adapter->lock is only used in et131x_multicast(), which is eventually
called by network stack function __dev_set_rx_mode(). __dev_set_rx_mode()
is always called by (net_device *)dev->addr_list_lock hold, to protect from
concurrent access. So adapter->lock is redundant.

Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Acked-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>