[crypto] Fix conditional subtraction in p384_key_from_seed

[pqcfox]

This small PR removes a flag clear in p384_keygen_from_seed.s to re-enable a conditional subtraction in p384_key_from_seed. Without this conditional subtraction, a small bias is introduced into the generated key distribution as the modified function will no longer always compute d = (d0' + d1') mod n for d0', d1' congruent to d0, d1 modulo n respectively.

For side channel purposes, the existing bn.xor after the conditional subtraction already handles clearing flags, so no further changes are needed.

[andreaskurth]

Hi @pqcfox,

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