staging: wilc1000: remove code for HOST_IF_MSG_GET_CHNL

The function that sent this message id was previously removed.
Finish the cleanup by removing the call to the message handler,
the message handler, the #define, and the channel var.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>