net: ethernet: mediatek: fix flow control settings on GMAC0 is not being enabled properly

Commit 08ef55c6
("net-next: mediatek: fix gigabit and flow control advertisement")
had supported proper flow control settings for GMAC1. But for GMAC0,

1.GMAC0 shares the common logic with GMAC1 inside mtk_phy_link_adjust()
to adapt various settings for the target phy.

2.GMAC0 uses fixed-phy to connect to a builtin gigabit switch with
fixed link speed as commit 0c72c50f


("net-next: mediatek: add fixed-phy support") describes.

3.However, fixed-phy doesn't enable SUPPORTED_Pause & SUPPORTED_Asym_Pause
supported flag on default that would cause mtk_phy_link_adjust() not to
enable flow control setting on GMAC0 properly and cause packet dropped
when high traffic.

Due to these reasons, the patch adds SUPPORTED_Pause & SUPPORTED_Asym_Pause
supported flags on fixed-phy used by the driver to have proper handling on
the both GMAC with the shared common logic.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>