net: bcmgenet: cleanup for bcmgenet_xmit()

1. Readability: Move nr_frags assignment a few lines down in order
   to bundle index -> ring -> txq calculations together.
2. Readability: Add parentheses around nr_frags + 1.
3. Minor fix: Stop the Tx queue and throw the error message only if
   the Tx queue hasn't already been stopped.

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