ftdi: address ftdi_usb_purge_buffers() deprecation in libftdi 1.5

ftdi: address ftdi_usb_purge_buffers() deprecation in libftdi 1.5

The ftdi_usb_purge_buffers() implementation suffered from an incorrect
implementation (inverse semantics). Library version 1.5 introduced a set
of substitute routines, and deprecated the existing purge API. Parameters
are identical, so we get away with a mere replacement of the routine name
which gets invoked. Which simplifies call sites, and concentrates all the
version dependency cruft in a central spot. The fact that all drivers
always flushed RX and TX and never individually also helped.