net: if: Assert that link layer addr was set by driver

There's a lot of code which assumes net_if_get_link_addr(iface)->addr
Forgetting to set it leads to deferred, spectacular crashes. It's
impractical to assert it on every usage. So, instead let's assert
it after call to driver->init(), as that is supposed to set it.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>