netxen_nic: use spin_[un]lock_bh around tx_clean_lock

While testing this driver with DEBUG_LOCKDEP and DEBUG_SPINLOCK
enabled did not produce any traces, it would be more prudent in the
case of tx_clean_lock to use spin_[un]lock_bh, since this lock is
manipulated in both the process and softirq contexts.

This patch was tested for functionality and regressions with netperf
and DEBUG_LOCKDEP and DEBUG_SPINLOCK enabled.

Signed-off-by: Tony Camuso <tcamuso@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>