rvp: Fix packed multiply-subtract operation logic

Corrects swapped add/subtract operators in packed 2-way multiply
loop. Spec requires (p0 - p1) but implementation computed (p1 - p0).
Even elements should add, odd elements should subtract.

Fixed instructions:
- PM2WSUB.H: P-ext spec
- PM2WSUBA.H: P-ext spec

Changed: Swapped += and -= for (j & 1) conditional

Signed-off-by: Max Chou <max.chou@sifive.com>

riscv/insns/pm2wsub_h.h

@@ -1,7 +1,7 @@
 require_rv32;
 P_WIDEN_REDUCTION_LOOP(32, 16, false, false, {
   if (j & 1)
-    p_res += p_rs1 * p_rs2;
-  else
     p_res -= p_rs1 * p_rs2;
+  else
+    p_res += p_rs1 * p_rs2;
 })

riscv/insns/pm2wsuba_h.h

@@ -1,7 +1,7 @@
 require_rv32;
 P_WIDEN_REDUCTION_LOOP(32, 16, true, false, {
   if (j & 1)
-    p_res += p_rs1 * p_rs2;
-  else
     p_res -= p_rs1 * p_rs2;
+  else
+    p_res += p_rs1 * p_rs2;
 })