cxgb4: Disable interrupts and napi before unregistering netdev

Disable interrupts and quiesce rx before unregistering net device to avoid crash
while unloading driver when traffic is flowing through.

Based on original work by Shameem Khalid <shameem@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>