iwlwifi: mvm: pre-initialize alive_data in wait_alive()

The function we pass to the wait alive notification procedure may may
not even get called if the timeout occurs before the function is
called.  To prevent accessing unitialized data in alive_data, pre-set
it to zero in the declaration.  Found by static analyzers.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>