staging: wilc1000: change handle_set_wfi_drv_handler's return type to void

When handle_set_wfi_drv_handler is called in hostIFthread that is a
kernel thread, it is not checked return type of this function. This
patch changes return type to void and removes braces if statement due to
have a single statement.

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>