[dv/hmac] Fix xcelium error
[dv/hmac] Fix xcelium error

File in issue #2404. Xcelium detected an error happened when a race
condition between msg_q.delete() and hmac_process. This PR addresses
this issue by delete msg_q after hmac_process is detected in the forever
loop.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@google.com>

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