dpaa_eth: avoid uninitialized variable false-positive warning

We can now build this driver on ARM, so I ran into a randconfig build
warning that presumably had existed on powerpc already.

drivers/net/ethernet/freescale/dpaa/dpaa_eth.c: In function 'sg_fd_to_skb':
drivers/net/ethernet/freescale/dpaa/dpaa_eth.c:1712:18: error: 'skb' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I'm slightly changing the logic here, to make it obvious to the
compiler that 'skb' is always initialized.

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