wil6210: fix wrong index in wil_vring_free

When destroying Rx vring, branch for Rx used wrong Tx descriptor:
while SW context was taken for "head", HW descriptor was, by mistake,
taken from "tail"

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>