netxen: Fix BUG "sleeping function called from invalid context"

o __netxen_nic_down() function might sleep while holding spinlock_t(tx_clean_lock).
  Acquire this lock for only releasing TX buffers instead of taking it
  for whole down path.

Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>