atm: eni: fix uninitialized variable warning

With -O3, gcc has found an actual unintialized variable stored
into an mmio register in two instances:

drivers/atm/eni.c: In function 'discard':
drivers/atm/eni.c:465:13: error: 'dma[1]' is used uninitialized in this function [-Werror=uninitialized]
   writel(dma[i*2+1],eni_dev->rx_dma+dma_wr*8+4);
             ^
drivers/atm/eni.c:465:13: error: 'dma[3]' is used uninitialized in this function [-Werror=uninitialized]

Change the code to always write zeroes instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>