wil6210: fix memory leak in the AP flow

When switching between STA and AP modes, memory allocated for Rx vring leaks
This is because start_ap() allocates Rx vring but stop_ap() do not free it.
Logically, Rx vring is not valid (HW can't use it anymore), so free it in reset()
Also, check double init for Rx vring and bail out with -EINVAL

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