net/macb: Factor out one-time assignment from loop

In 02c958dd (net/macb: add TX multiqueue support for gem), the
initialization of tx_head and tx_tail in macb_init_rings() was moved
inside the loop that iterates over each element in the ring.  Since
tx_head and tx_tail only need to be assigned once, move them back out of
the loop.

Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>