bnxt_en: Don't use rtnl lock to protect link change logic in workqueue.

As a further improvement to the PF/VF link change logic, use a private
mutex instead of the rtnl lock to protect link change logic.  With the
new mutex, we don't have to take the rtnl lock in the workqueue when
we have to handle link related functions.  If the VF and PF drivers
are running on the same host and both take the rtnl lock and one is
waiting for the other, it will cause timeout.  This patch fixes these
timeouts.

Fixes: 90c694bb ("bnxt_en: Fix RTNL lock usage on bnxt_update_link().")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>