net: stmmac: Fix possible deadlock when disabling EEE support

When stmmac_eee_init() is called to disable EEE support, then the timer
for EEE support is stopped and we return from the function. Prior to
stopping the timer, a mutex was acquired but in this case it is never
released and so could cause a deadlock. Fix this by releasing the mutex
prior to returning from stmmax_eee_init() when stopping the EEE timer.

Fixes: 74371272

 ("net: stmmac: Convert to phylink and remove phylib logic")
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>