bgmac: simplify tx ring index handling

Keep incrementing ring->start and ring->end instead of pointing it to
the actual ring slot entry. This simplifies the calculation of the
number of free slots.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>