net: dsa: sja1105: remove set but not used variable 'prev_time'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/dsa/sja1105/sja1105_vl.c:468:6: warning: variable ‘prev_time’ set but not used [-Wunused-but-set-variable]
  u32 prev_time = 0;
      ^~~~~~~~~
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/sja1105/sja1105_vl.c

@@ -465,7 +465,6 @@ sja1105_gating_cfg_time_to_interval(struct sja1105_gating_config *gating_cfg,
 	struct sja1105_gate_entry *last_e;
 	struct sja1105_gate_entry *e;
 	struct list_head *prev;
-	u32 prev_time = 0;
 
 	list_for_each_entry(e, &gating_cfg->entries, list) {
 		struct sja1105_gate_entry *p;
@@ -476,7 +475,6 @@ sja1105_gating_cfg_time_to_interval(struct sja1105_gating_config *gating_cfg,
 			continue;
 
 		p = list_entry(prev, struct sja1105_gate_entry, list);
-		prev_time = e->interval;
 		p->interval = e->interval - p->interval;
 	}
 	last_e = list_last_entry(&gating_cfg->entries,