avdevice/decklink: Fix segfault when running -list_devices on OSX

The string is allocated with CFStringGetCString but was being
deallocated with free(), which would intermittently result in
a segmentation fault.  Use the correct function for freeing the
allocated CFString.

Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>