On interrupt handler, save the actual data read from mbox.  The previous
macro wasn't needed and was being used with swapped arguments which always
give the same result (0) defeating the overflow check.

On initialization, do not use bcm_mbox_intr() to read the pending messages,
with the new semaphore based implementation this will lead to semaphore
being incremented on the channels that contain pending data and will make
the first read for that channel return stale data.

This fixes the hang that happens on boot while initializing the cpufreq on
Raspberry Pi.
Remove the '#undef DEBUG' that should not be committed.