ti-img: fix WHENCE entry for Imagination D5520 Decoder Firmware

We accidentally used the file name rather than the full path to
Imagination's D5520 Decoder firmware breaking 'make check'

Update to the full path to the firmware binary

Fixes: 783c4260

 ("Add: add firmware for Imagination D5520 Decoder")
Signed-off-by: Bryan Brattlof <bb@ti.com>
Acked-by: Darren Etheridge <detheridge@ti.com>