mvpp2: use correct size for memset

gcc-7 detects a short memset in mvpp2, introduced in the original
merge of the driver:

drivers/net/ethernet/marvell/mvpp2.c: In function 'mvpp2_cls_init':
drivers/net/ethernet/marvell/mvpp2.c:3296:2: error: 'memset' used with length equal to number of elements without multiplication by element size [-Werror=memset-elt-size]

The result seems to be that we write uninitialized data into the
flow table registers, although we did not get any warning about
that uninitialized data usage.

Using sizeof() lets us initialize then entire array instead.

Fixes: 3f518509 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>