Fix an incorrect format specifier in printf

This was causing clang to complain:

buffer_hub_queue_producer.cpp:137:28: warning: format specifies type
'unsigned long long' but the argument has type 'uint64_t' (aka 'unsigned
long')

Bug: None
Test: mma. Warning is gone
Change-Id: I578c5a910727a578557777ca531ea17a3c90cee8