Fix unused variable warning in fwohci.c

With clang 15, the following -Werror warning is produced:

sys/dev/firewire/fwohci.c:2762:23: error: variable 'pcnt' set but not used [-Werror,-Wunused-but-set-variable]
        int len, plen, hlen, pcnt, offset;
                             ^

The 'pcnt' variable is eventually used only in an #if 0'd block,
obviously meant for debugging. Ensure that 'pcnt' is only declared and
used when COUNT_PACKETS is defined, so the debugging can be easily
turned on later, if desired.

MFC after: 3 days