crypto: sahara - Staticize local symbol

This local symbol is used only in this file.
Fix the following sparse warnings:

drivers/crypto/sahara.c:420:6: warning: symbol 'sahara_watchdog' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>