netdev: bfin_mac: fix memory leak when freeing dma descriptors

The size of the desc array is not the size of the desc structure, so
when we try to free up things, we leak some parts.

Reported-by: Regis Dargent <rdargent@edevice.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>