[entropy_src/rtl] return ack sm to idle when disabled
[entropy_src/rtl] return ack sm to idle when disabled

In the case the entropy_src is disabled and a request from CSRNG is pending, the state machine should return to idle and wait to be enabled again.
Fixes #10133.

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

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