ft2232: remove ft2232_large_scan memory leak

This is a very long outstanding issue see:
http://lists.berlios.de/pipermail/openocd-development/2011-June/019404.html

As this driver is deprecated the fix is added to purely to reduce the warnings
reported by clang.

Change-Id: I3a16a704e0e8db27efda50fdcfdd35abf5ebed0f
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1278
Tested-by: jenkins