drivers: wifi: Use mutex for spinlock

Ideally we should be using Zephyr spinlock APIs but that requires
changes to shim API, so, for this maintenance release just replace with
mutex to keep the context same and no API changes.

This solves the locking issue that we see when control and data path are
excited concurrently due to locking semantics of semaphores, the issue
is not root caused but mutex enforce strict locking semantics for
multiple threads and solve the issue.

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