net/pasemi: fix compiler warning

Fix this compiler warning (on PowerPC) by not marking a parameter as
const:

drivers/net/ethernet/pasemi/pasemi_mac.c: In function 'pasemi_mac_replenish_rx_ring':
drivers/net/ethernet/pasemi/pasemi_mac.c:646:3: warning: passing argument 1 of 'netdev_alloc_skb' discards qualifiers from pointer target type
include/linux/skbuff.h:1706:31: note: expected 'struct net_device *' but argument is of type 'const struct net_device *'

Cc: Olof Johansson <olof@lixom.net>
Cc: Pradeep A. Dalvi <netdev@pradeepdalvi.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>