net: bcmgenet: simplify bcmgenet_init_dma()

Do the two kcalloc() calls first, before proceeding into Rx/Tx DMA init.
Makes the error case handling much simpler.

Signed-off-by: Petri Gynther <pgynther@google.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Jaedon Shin <jaedon.shin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>