Revert "modules: nrf_wifi: Use spinlocks"

This reverts commit 3e9dffbe.
Though this works fine, when CONFIG_ASSERT=y the spinlock validation
fails as the underlying code though uses OSAL spinlock APIs is not ready

* sleeping with spinlock held
* multiple threads taking the same spinlock (might work on UP, but not
  on SMP on the same CPU)

Revert this for now, till the underyling is robust.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>