To clarify: what SirDice probably tries to say (and he's absolutely right about that) is that driver and kernel code should not use userland libraries. However, if (some form of) stdarg is also available in the kernel source tree then it's perfectly fine to use that; just include the right header file and hack away.
Fonz
P.S. If I'm not mistaken, stdarg consists solely of macros, so in this case there shouldn't be any problems as long as one includes the right header file.