drivers: wifi: simplelink: Move NWP configuration to iface_init

Previously, the NWP (Network Coprocessor) was configured during
simplelink_init().

Since the SimpleLink "FastConnect" feature was added,
meaning an automatic reconnect can occur at this time, this
results in early callbacks into the network stack
before it was fully initialized, resulting in a NULL iface
ptr being sent in as argument.

Now, for safety, the NWP startup is moved later to iface_init,
where it more logically belongs.

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>