crypto: omap-sham - fix digcnt register handling with export/import

Running export/import for hashes in peculiar order (mostly done by
openssl) can mess up the internal book keeping of the OMAP SHA core.
Fix by forcibly writing the correct DIGCNT back to hardware. This issue
was noticed while transitioning to openssl 1.1 support.

Fixes: 0d373d60 ("crypto: omap-sham - Add OMAP4/AM33XX SHAM Support")
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>