tg3: tg3_alloc_rx_skb(tnapi => tp)

This patch converts the tnapi argument of tg3_alloc_rx_skb() to tp.  The
level of indirection is unnecessary.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>