ftdi: Fix error in calloc retval check

Found by clang.

Change-Id: Iaa952b063779f4d392d8bdc62a3e7ce9daef2740
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1498
Tested-by: jenkins
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>