net: sxgbe: fix potential null dereference

This fixes following:

drivers/net/ethernet/samsung/sxgbe/sxgbe_main.c:1828 sxgbe_hw_init()
error: potential null dereference 'priv->hw'.  (kmalloc returns null)

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Byungho An <bh74.an@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>