wil6210: prevent platform callbacks after uninit

After calling platform_ops.uninit() it is still possible to invoke
platform callbacks.
To prevent this, zero platform_ops right after invoking uninit.

Signed-off-by: Dedy Lansky <qca_dlansky@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>