GD SMP ECC: Add const qualifier to ECC_PointMult

Second parameter passed to ECC_PointMult is either remote device public
key, or the constant from the BT spec. Both of them should never be modified.

Assigning the "z" value inside ECC_PointMult had no effect on the rest
of computation.

Test: SmpEccValidationTest
Bug: 126463795
Change-Id: I0530d26e0e8841cb8b263ea948ae7a19850f0670