[entropy_src/rtl] Prevent interrupt status for data valid
[entropy_src/rtl] Prevent interrupt status for data valid

For the mode where the entropy read register is not being used,
the related FIFO state and interrupt status bits should be unchanging.

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

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