net: bgmac: Fix infinite loop in bgmac_dma_tx_add()

Nothing is decrementing the index "i" while we are cleaning up the
fragments we could not successful transmit.

Fixes: 9cde9450 ("bgmac: implement scatter/gather support")
Reported-by: coverity (CID 1352048)
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>