staging: wilc1000: Replace kthread with workqueue for host interface

Deconstruct the kthread / message_queue logic, replacing it with
create_singlethread_workqueue() / queue_work() setup, by adding a
'struct work_struct' to 'struct host_if_msg'. The current kthread
hostIFthread() is converted to a work queue helper with the name
'host_if_work'.
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>

drivers/staging/wilc1000/TODO

@@ -4,6 +4,11 @@ TODO:
 - remove custom debug and tracing functions
 - rework comments and function headers(also coding style)
 - replace all semaphores with mutexes or completions
+- Move handling for each individual members of 'union message_body' out
+  into a separate 'struct work_struct' and completely remove the multiplexer
+  that is currently part of host_if_work(), allowing movement of the
+  implementation of each message handler into the callsite of the function
+  that currently queues the 'host_if_msg'.
 - make spi and sdio components coexist in one build
 - turn compile-time platform configuration (BEAGLE_BOARD,
   PANDA_BOARD, PLAT_WMS8304, PLAT_RKXXXX, CUSTOMER_PLATFORM, ...)