spapr_drc: Fix potential undefined behaviour

The DRC_INDEX_ID_MASK macro does a left shift on ~0, which is a signed
quantity, and therefore undefined behaviour according to the C spec.  In
particular this causes warnings from the clang sanitizer.

This fixes it by calculating the same mask without using ~0 (I think the
new method is a more common idiom for generating masks anyway).  For good
measure I also use 1ULL to force the expression's type to unsigned long
long, which should be good for assigning to anything we're going to want
to.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
