net: bcmgenet: fix TX reclaim accounting for fragments

The GENET driver supports SKB fragments, and succeeds in transmitting
them properly, but when reclaiming these transmitted fragments, we will
only update the count of free buffer descriptors by 1, even for SKBs
with fragments. This leads to the networking stack thinking it has more
room than the hardware has when pushing new SKBs, and backing off
consequently because we return NETDEV_TX_BUSY.

Fix this by accounting for the SKB nr_frags plus one (itself) and update
ring->free_bds accordingly with that value for each iteration loop in
__bcmgenet_tx_reclaim().

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