sh_eth: fix uninitialized arrays in sh_eth_ring_init()

sh_eth_ring_free()  called in the sh_eth_ring_init()'s error path expects
the arrays pointed  to  by 'sh_eth_private::[rt]x_skbuff' to be initialized
with NULLs but they are allocated with just kmalloc_array() and so are left
filled with random data. Use kcalloc() instead.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>