[otbn] Start WSR numbers at zero
[otbn] Start WSR numbers at zero

For no obvious reason (and I wrote that part), WSR number start at 1.
Change that to start at 0. This is in line with the RISC-V CSR numbers.
After all, OTBN isn't written in Matlab.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

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