net: b53: enable BPDU reception for management port

For STP to work, receiving BPDUs is essential, but the appropriate bit
was never set. Without GC_RX_BPDU_EN, the switch chip will filter all
BPDUs, even if an appropriate PVID VLAN was setup.

Fixes: ff39c2d ("net: dsa: b53: Add bridge support")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://patch.msgid.link/20250414200434.194422-1-jonas.gorski@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: KanjiMonster

drivers/net/dsa/b53/b53_common.c

@@ -737,6 +737,15 @@ static void b53_enable_mib(struct b53_device *dev)
 	b53_write8(dev, B53_MGMT_PAGE, B53_GLOBAL_CONFIG, gc);
 }
 
+static void b53_enable_stp(struct b53_device *dev)
+{
+	u8 gc;
+
+	b53_read8(dev, B53_MGMT_PAGE, B53_GLOBAL_CONFIG, &gc);
+	gc |= GC_RX_BPDU_EN;
+	b53_write8(dev, B53_MGMT_PAGE, B53_GLOBAL_CONFIG, gc);
+}
+
 static u16 b53_default_pvid(struct b53_device *dev)
 {
 	if (is5325(dev) || is5365(dev))
@@ -876,6 +885,7 @@ static int b53_switch_reset(struct b53_device *dev)
 	}
 
 	b53_enable_mib(dev);
+	b53_enable_stp(dev);
 
 	return b53_flush_arl(dev, FAST_AGE_STATIC);
 }