net: bcmgenet: simplify __bcmgenet_tx_reclaim()

1. Use c_index and ring->c_index to determine how many TxCBs/TxBDs are
   ready for cleanup
   - c_index = the current value of TDMA_CONS_INDEX
   - TDMA_CONS_INDEX is HW-incremented and auto-wraparound (0x0-0xFFFF)
   - ring->c_index = __bcmgenet_tx_reclaim() cleaned up to this point on
     the previous invocation

2. Add bcmgenet_tx_ring->clean_ptr
   - index of the next TxCB to be cleaned
   - incremented as TxCBs/TxBDs are processed
   - value always in range [ring->cb_ptr, ring->end_ptr]

3. Fix incrementing of dev->stats.tx_packets
   - should be incremented only when tx_cb_ptr->skb != NULL

These changes simplify __bcmgenet_tx_reclaim(). Furthermore, Tx ring size
can now be any value.

With the old code, Tx ring size had to be a power-of-2:
   num_tx_bds = ring->size;
   c_index &= (num_tx_bds - 1);
   last_c_index &= (num_tx_bds - 1);

Signed-off-by: Petri Gynther <pgynther@google.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>