net: stmmac: fix ethtool flow control not able to get/set

Currently ethtool was not able to get/set the flow control due to a
missing "!". It will always return -EOPNOTSUPP even the device is
flow control supported.

This patch fixes the condition check for ethtool flow control get/set
function for ETHTOOL_LINK_MODE_Asym_Pause_BIT.

Fixes: 3c1bcc86 (“net: ethernet: Convert phydev advertize and supported from u32 to link mode”)
Signed-off-by: Tan, Tee Min <tee.min.tan@intel.com>
Reviewed-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Voon, Weifeng <weifeng.voon@intel.com@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>