net: bcmgenet: Fix unmapping of fragments in bcmgenet_xmit()

In case we fail to map a single fragment, we would be leaving the
transmit ring populated with stale entries.

This commit introduces the helper function bcmgenet_put_txcb()
which takes care of rewinding the per-ring write pointer back to
where we left.

It also consolidates the functionality of bcmgenet_xmit_single()
and bcmgenet_xmit_frag() into the bcmgenet_xmit() function to
make the unmapping of control blocks cleaner.

Fixes: 1c1008c7 ("net: bcmgenet: add main driver file")
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>