#37896: Fix silu_init for BH (#37897)

### Ticket
#37896
#37871

### Problem description
Silu init was incorrectly toggling `_init_sfpu_reciprocal_` based on
approx mode flag, but silu is hardcoded to use non-approx sigmoid. We
shouldn't be duplicating the recip init call in sigmoid since that is
error prone, and should just invoke sigmoid init in non-approx mode.

### What's changed
Change silu init to call `sigmoid_init<false>();`

### Checklist

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[Demo](https://github.com/tenstorrent/tt-metal/actions/workflows/galaxy-demo-tests.yaml)
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Author: tt-aho

tt_metal/hw/ckernels/blackhole/metal/llk_api/llk_sfpu/ckernel_sfpu_silu.h

@@ -29,11 +29,8 @@ inline void calculate_silu() {
 
 template <bool APPROXIMATION_MODE>
 inline void silu_init() {
-    if constexpr (!APPROXIMATION_MODE) {
-        _init_sfpu_reciprocal_<false>();
-    } else {
-        _init_sfpu_reciprocal_<true>();
-    }
+    // calculate_silu uses the non-approx sigmoid path via _sfpu_sigmoid_, so we must use non-approx sigmoid_init
+    sigmoid_init<false>();
 }
 
 }  // namespace ckernel::sfpu