net: ethernet: mediatek: fix warning in phy_start_aneg

net: ethernet: mediatek: fix warning in phy_start_aneg

linux 5.0-rc1 shows following warning on bpi-r2/mt7623 bootup:

[ 5.170597] WARNING: CPU: 3 PID: 1 at drivers/net/phy/phy.c:548 phy_start_aneg+0x110/0x144
[ 5.178826] called from state READY
....
[ 5.264111] [<c0629fd4>] (phy_start_aneg) from [<c0e3e720>] (mtk_init+0x414/0x47c)
[ 5.271630] r7:df5f5eec r6:c0f08c48 r5:00000000 r4:dea67800
[ 5.277256] [<c0e3e30c>] (mtk_init) from [<c07dabbc>] (register_netdevice+0x98/0x51c)
[ 5.285035] r8:00000000 r7:00000000 r6:c0f97080 r5:c0f08c48 r4:dea67800
[ 5.291693] [<c07dab24>] (register_netdevice) from [<c07db06c>] (register_netdev+0x2c/0x44)
[ 5.299989] r8:00000000 r7:dea2e608 r6:deacea00 r5:dea2e604 r4:dea67800
[ 5.306646] [<c07db040>] (register_netdev) from [<c06326d8>] (mtk_probe+0x668/0x7ac)
[ 5.314336] r5:dea2e604 r4:dea2e040
[ 5.317890] [<c0632070>] (mtk_probe) from [<c05a78fc>] (platform_drv_probe+0x58/0xa8)
[ 5.325670] r10:c0f86bac r9:00000000 r8:c0fbe578 r7:00000000 r6:c0f86bac r5:00000000
[ 5.333445] r4:deacea10
[ 5.335963] [<c05a78a4>] (platform_drv_probe) from [<c05a5248>] (really_probe+0x2d8/0x424)

maybe other boards using this generic driver are affected

v2:
optimization:

- phy_set_max_speed() is only needed if you want to reduce the
  max speed, typically if the PHY supports 1Gbps but the MAC
  supports 100Mbps only.

- The pause parameters are autonegotiated. Except you have a specific
  need you normally don't need to manually fiddle with this.

- phy_start_aneg() is called implicitly by the phylib state machine,
  you shouldn't call it manually except you have a good excuse.

- netif_carrier_on/netif_carrier_off in mtk_phy_link_adjust() isn't
  needed. It's done by phy_link_change() in phylib.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>