staging: wilc1000: Replace semaphore txq_add_to_head_cs with mutex

The semaphore 'txq_add_to_head_cs' is a simple mutex, so it should be
written as one. Semaphores are going away in the future. Also, removing
the timeout scenario as the error handling code does not propagate the
timeout properly.

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>