ecp.c: Refactored a minor check in ecp check privkey

[minosgalanakis]

Description

Added a comment based on an investigation we did at the functional behaviour of a check.

PR checklist

Please remove the segment/s on either side of the | symbol as appropriate, and add any relevant link/s to the end of the line. If the provided content is part of the present PR remove the # symbol.

• [x] changelog not required because: is a minor non functional change.
• [x] development PR provided
• [x] framework PR not required
• [x] 3.6 PR provided #9495
• [x] 2.28 PR provided #9496
• tests not required because: is a minor non functional change.

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