lan743x: fix "BUG: invalid wait context" when setting rx mode

In the net core, the struct net_device_ops -> ndo_set_rx_mode()
callback is called with the dev->addr_list_lock spinlock held.

However, this driver's ndo_set_rx_mode callback eventually calls
lan743x_dp_write(), which acquires a mutex. Mutex acquisition
may sleep, and this is not allowed when holding a spinlock.

Fix by removing the dp_lock mutex entirely. Its purpose is to
prevent concurrent accesses to the data port. No concurrent
accesses are possible, because the dev->addr_list_lock
spinlock in the core only lets through one thread at a time.

Fixes: 23f0703c

 ("lan743x: Add main source files for new lan743x driver")
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Link: https://lore.kernel.org/r/20201109203828.5115-1-TheSven73@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>