alx: take rtnl before calling __alx_open from resume

The __alx_open function can be called from ndo_open, which is called
under RTNL, or from alx_resume, which isn't. Since commit d768319c,
we're calling the netif_set_real_num_{tx,rx}_queues functions, which
need to be called under RTNL.

This is similar to commit 0c2cc02e ("igb: Move the calls to set the
Tx and Rx queues into igb_open").

Fixes: d768319c ("alx: enable multiple tx queues")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>