staging: wilc1000: Replace semaphore txq_event with completion

The semaphore 'txq_event' is used as completion, so convert it
to a struct completion type.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>