net: bcmgenet: cleanup ring interrupt masking and unmasking

Since the NAPI interrupts are basically ignored when NAPI is
disabled we don't need to mask them within the functions
bcmgenet_disable_tx_napi() and bcmgenet_disable_rx_napi().
So wait until all NAPI instances are disabled and mask all of the
bcmgenet driver interrupts together in bcmgenet_netif_stop().

The interrupts can still be enabled in the functions
bcmgenet_enable_tx_napi() and bcmgenet_enable_rx_napi(), but use
the ring context int_enable() method to keep the functionality
consistent and the code cleaner.

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