crypto: omap-sham - irq and dma handling changes

It could be a situation, that tasklet is executed twice because of
certain delay between dma callback and irq handler execution.
In that case, second tasklet execution could actually corrupt the data
of the new started dma transactions.

This patch improves tasklet logic and prevents above described cases.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>