[otbn] fix function call in ISS
[otbn] fix function call in ISS

Order of arguments of call to set_half_word_unsigned()
has to be reversed to match expectation.

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

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