[otp] Cast between different widths more explicitly in otp_ctrl_dai
[otp] Cast between different widths more explicitly in otp_ctrl_dai

The existing code was causing Verilator lint errors and this patch
makes the casts explicit, adding comments to explain why they are
right and adding a couple of assertions to make sure nothing comes
unstuck.

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

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