net: bcmgenet: relax lock constraints to reduce IRQ latency

Since the ring locks are not used in a hard IRQ context it is often
not necessary to disable global IRQs while waiting on a lock.

Using less restrictive lock and unlock calls improves the real-time
responsiveness of the system.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>