ath5k: properly free rx dma descriptors

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>