sm_io/*/acq/*: fix (again) number of pre/post acquisition samples

We need to acquire a multiple number of
samples_alignment value. With samples_alignment
being 4 (typical for the ADC and ADCSWAP channels),
the following equation yields correct results:

num_samples_aligned = num_samples_pre + num_samples_aligned -
        (num_samples_pre % num_samples_aligned)

src/sm_io/modules/acq/sm_io_acq_exp.c

@@ -161,8 +161,10 @@ static int _acq_data_acquire (void *owner, void *args, void *ret)
      * acquisition channel sample size */
     uint32_t samples_alignment =
         DDR3_PAYLOAD_SIZE/acq->acq_buf[chan].sample_size;
-    uint32_t num_samples_pre_aligned = num_samples_pre + (num_samples_pre % samples_alignment);
-    uint32_t num_samples_post_aligned = num_samples_post + (num_samples_post % samples_alignment);
+    uint32_t num_samples_pre_aligned = num_samples_pre + samples_alignment -
+        (num_samples_pre % samples_alignment);
+    uint32_t num_samples_post_aligned = num_samples_post + samples_alignment -
+        (num_samples_post % samples_alignment);
 
     /* Set the parameters: number of samples of this channel */
     acq->acq_params[chan].num_samples_pre = num_samples_pre_aligned;