[hmac,lint] Use a static cast for RoundWidth in sha2.sv
[hmac,lint] Use a static cast for RoundWidth in sha2.sv

This avoids a Verilator width mismatch error.

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

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