[otbn,dv] Use check_arg_count consistently in stepped.py
[otbn,dv] Use check_arg_count consistently in stepped.py

A trivial nit fix, but it actually removes quite a bit of boilerplate!
Also break some long lines.

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

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