net: bcmgenet: manage dma interrupts in napi code

This commit moves DMA interrupt enabling out of init_umac() and adds
the masking of these interrupts to the napi enable and disable code.

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