Blackfin: fix jtag console tstc

The jtag tstc operation was checking the hardware to see if data is
available from it (which is fine for the jtag getc operation), but the
higher layers need to know whether any data is available.  Since we have
to read up to 4 bytes at a time from the hardware, the higher layers need
to know they can consume the cached bytes as well.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>