wmi: Remove private %pUL implementation

The work performed by wmi_gtoa is equivalent to simply sprintf(out,
"%pUL", in), so one could replace its body by this. However, most
users feed the result directly as a %s argument to some other function
which also understands the %p extensions (they all ultimately use
vsnprintf), so we can eliminate some stack buffers and quite a bit of
code by just using %pUL directly.

In wmi_dev_uevent I'm not sure whether there's room for a
nul-terminator in env->buf, so I've just replaced wmi_gtoa with the
equivalent sprintf call.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>