When freeing rx dma descriptors, use the right buffer size.
Fixes kernel oopses on module unload on ixp4xx and most likely
other platforms as well.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>