[hmac,dv] Don't use streaming concatenation in a ternary
[hmac,dv] Don't use streaming concatenation in a ternary

This isn't technically allowed by the IEEE 1800 LRM (see the start of
section 11.4.14). In particular Xcelium doesn't allow it! So tweak
things so that we only use it as the full right hand side of an
assignment.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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