drivers: wifi: Fix mutex re-initialization

The mutex is used to protect RPU zephyr context which gets modified for
every interface down and up (including recovery), so, it was being
re-initialized but also used to protect down and up which is a bug.

Move the re-initialization to the driver entry so that it happens only
once and we can properly use the mutext for down and up protection.

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