mv643xx_eth: inline txq_alloc_desc_index()

Since txq_alloc_desc_index() is a very simple function, and since
descriptor ring index handling for transmit reclaim, receive
processing and receive refill is already handled inline as well,
inline txq_alloc_desc_index() into its two call sites.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>