staging: bcm2835-codec: Fix imbalance in dma_buf_get/dma_buf_put

When represented with a dmabuf buffer that had previously been
imported, there was a call to dma_buf_get without a matching
dma_buf_put. This left dmabufs in limbo after all users had
supposedly released them.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>