staging: comedi: Use '"%s:", __func__' instead of function name

Replace all occurrences of functions' names in strings by a reference
to __func__, to improve robustness. Problem found with checkpatch.

Signed-off-by: Eisha Chen-yen-su <chenyensu0@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>