[csrng/rtl] fix for dif test
[csrng/rtl] fix for dif test

Fix will clear out any residual seed info when doing KAT.
Discovered at full chip where ES does a normal boot sequence first.

Signed-off-by: Mark Branstad <d7ca710cc6ec5dc810c866dae5369b19445fbeef@wdc.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan