Use 8x1 SpMM micro-kernel on WebAssembly

PiperOrigin-RevId: 276525021
diff --git a/src/init.c b/src/init.c
index d67cb87..baff93e 100644
--- a/src/init.c
+++ b/src/init.c
@@ -999,8 +999,8 @@
     };
     #ifndef XNN_NO_SPNCHW_OPERATORS
       xnn_params.f32.spmm = (struct spmm_parameters) {
-        .ukernel = (xnn_spmm_ukernel_function) xnn_f32_spmm_ukernel_4x1__scalar,
-        .mr = 4,
+        .ukernel = (xnn_spmm_ukernel_function) xnn_f32_spmm_ukernel_8x1__scalar,
+        .mr = 8,
         .nr = 1,
       };
       xnn_params.f32.hwc2spchw_dconv3x3c3s2 = (struct hwc2spchw_dconv_parameters) {