dwc_eth_qos: do not clear pause flags from phy_device->supported

phy_device->supported is originally set by the PHY driver.
The ethernet driver should filter phy_device->supported to only contain
flags supported by the IP.
The IP supports setting rx and tx flow control independently,
therefore SUPPORTED_Pause and SUPPORTED_Asym_Pause should not be cleared.
If the flags are cleared, pause frames cannot be enabled (even if they
are supported by the PHY).

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Jesper Nilsson <jespern@axis.com>
Acked-by: Lars Persson <larper@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>