crypto: omap-sham - Only release DMA channel if successfully requested

In omap_sham_probe() and omap_sham_remove(), 'dd->dma_lch'
is released without checking to see if it was successfully
requested or not.  This is a bug and was identified and
reported by Dan Carpenter here:

	http://www.spinics.net/lists/devicetree/msg11023.html

Add code to only release 'dd->dma_lch' when its not NULL
(that is, when it was successfully requested).

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
CC: Joel Fernandes <joelf@ti.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>