sky2: Fix crash inside sky2_rx_clean

If sky2->tx_le = pci_alloc_consistent() or sky2->tx_ring = kcalloc() in
sky2_alloc_buffers() fails, sky2->rx_ring = kcalloc() will never be called.
In this error case handling, sky2_rx_clean() is called from within
sky2_free_buffers().

In sky2_rx_clean() we find the following:

...
   memset(sky2->rx_le, 0, RX_LE_BYTES);
...

This results in a memset using a NULL pointer and will crash the system.

Signed-off-by: Mirko Lindner <mlindner@marvell.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>