libftdi: enable async mode

Apparently, async mode for libftdi 0.20 is allegedly disabled when using
libusb-compat wrappers, as libftdi does not really support libftdi 1.x. Because
we only ship libusb-compat, this would normally make async mode completely
unavailable.

Except distributions like Ubuntu just disable this check completely! See this
patch from Launchpad:

https://bazaar.launchpad.net/~ubuntu-branches/ubuntu/trusty/libftdi/trusty/view/head:/debian/patches/04_async_mode.diff



In the spirit of good competition (and feature parity for software that
*doesn't* support the synchronous mode, thanks to Ubuntu silently ensuring
their async-only paths work) we enable this just the same.

Signed-off-by: Austin Seipp <aseipp@pobox.com>