tg3: Add param short_preamble to enable MDIO traffic to external PHYs

Add parameter to enable *short preamble* for MAC, so MDIO access to some
external PHY, like BCM54616, can be validated.

Applies to the five platforms below, that have the ethernet controller
BCM5720:

as7116_54x, as7326_56x, as7716_32x-r0, as7716_32xb, and as7816_64x

Verified by mii-tool. (31 is the address of external phy).

    root@sonic:/home/admin# mii-tool -v eth0 -p 31
    using the specified MII index 31.
    eth0: negotiated, link ok
    product info: vendor 00:d8:97, model 17 rev 2
    basic mode: autonegotiation enabled
    basic status: autonegotiation complete, link ok
    capabilities: 1000baseT-HD 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
    advertising: flow-control
    link partner: 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD

Note, this upstreams a patch to the SONiC Linux kernel added in 2018
[1][2][3].

[1]: sonic-net/sonic-linux-kernel#71
[2]: sonic-net/sonic-linux-kernel@ad754bd
[3]: sonic-net/sonic-buildimage#2318

Signed-off-by: Roy Lee <roy_lee@accton.com>
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: NipaLocal <nipa@local>

drivers/net/ethernet/broadcom/tg3.c

@@ -233,6 +233,10 @@ static int tg3_debug = -1;	/* -1 == use TG3_DEF_MSG_ENABLE as value */
 module_param(tg3_debug, int, 0);
 MODULE_PARM_DESC(tg3_debug, "Tigon3 bitmapped debugging message enable value");
 
+static int short_preamble = 0;
+module_param(short_preamble, int, 0);
+MODULE_PARM_DESC(short_preamble, "Enable short preamble.");
+
 #define TG3_DRV_DATA_FLAG_10_100_ONLY	0x0001
 #define TG3_DRV_DATA_FLAG_5705_10_100	0x0002
 
@@ -1493,6 +1497,11 @@ static void tg3_mdio_config_5785(struct tg3 *tp)
 static void tg3_mdio_start(struct tg3 *tp)
 {
 	tp->mi_mode &= ~MAC_MI_MODE_AUTO_POLL;
+
+	if(short_preamble) {
+	    tp->mi_mode |= MAC_MI_MODE_SHORT_PREAMBLE;
+	}
+
 	tw32_f(MAC_MI_MODE, tp->mi_mode);
 	udelay(80);