acpi: Tasks: Use M_WAITOK to allocate the taskqueue

[olce]

Summary

Memory failure at this point normally does not happen except if the
kernel is running on a machine with a too tiny amount of memory, in
which case it is likely to panic or to be unusable anyway. However,
letting it panic here with a NULL pointer dereference is not very
helpful. Instead, use M_WAITOK, hoping the memory allocator itself will
indicate the low memory condition if it cannot succeed.