buildroot

libftdi: fix typo in variable name

As pointed out during the check-package discussion, there is a typo in a
variable name:

http://lists.busybox.net/pipermail/buildroot/2016-December/180765.html

Reported-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>