staging: unisys: visorbus: visorbus_main.c: use __func__ over hardcoded name

As reported by checkpatch.pl, replace hard-coded usage of the current
function's name in format string with usage of __func__.

Signed-off-by: Zachary Dremann <dremann@gmail.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: David Binder <david.binder@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>