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>
Git-commit: 4d1ac0721aae84eec3c8aa54c7574b14f79863bc
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Liron Kuch <lkuch@codeaurora.org>