[otbn,dv] Fix return type of _set_loop_counter in otbn_base_vseq
[otbn,dv] Fix return type of _set_loop_counter in otbn_base_vseq

If you forget to supply a return type, SystemVerilog defaults to bit,
not void.

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

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