Calculating the BD entry using a modulus operation isn't optimal, especially
inside the loop.  This patch removes the modulus operations in favour of:

i)  simply checking for wrapping in the case of cur_rx
ii) forcing num_tx to be a power of two and using it to mask out the
    entry from cur_tx

The also prevents possible issues related overflow of the cur_rx and cur_tx
counters.
Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: David S. Miller <davem@davemloft.net>