bgmac: simplify dma init/cleanup

Instead of allocating buffers at device init time and initializing
descriptors at device open, do both at the same time (during open).
Free all buffers when closing the device.

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