[dvsim] Correct bug in regression creation in dvsim's Modes.py
[dvsim] Correct bug in regression creation in dvsim's Modes.py

The create_regressions function is a static method, so there's no
'self' available. I think this should have been called on the
regression object.

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

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