net: stmmac: add ethtool support for get/set channels

Restructure NAPI add and delete process so that we can call them
accordingly in open() and ethtool_set_channels() accordingly.

Introduced stmmac_reinit_queues() to handle the transition needed
for changing Rx & Tx channels accordingly.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>