linux-user: fix old style decrement usage

Modern compilers do not parse "=-" as decrement:
you must use "-=" for that.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

linux-user/arm/nwfpe/fpa11.c

@@ -191,7 +191,7 @@ unsigned int EmulateAll(unsigned int opcode, FPA11* qfpa, CPUARMState* qregs)
   if(nRc == 1 && get_float_exception_flags(&fpa11->fp_status))
   {
     //printf("fef 0x%x\n",float_exception_flags);
-    nRc=-get_float_exception_flags(&fpa11->fp_status);
+    nRc -= get_float_exception_flags(&fpa11->fp_status);
   }
 
   //printf("returning %d\n",nRc);