cxgb4: get rid of custom busy poll code

In linux-4.5, busy polling was implemented in core
NAPI stack, meaning that all custom implementation can
be removed from drivers.

Not only we remove lot of code, we also remove one spin_lock()
from driver fast path.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>