net: phy: Add helper for advertise to lcl value

Add a helper to convert the local advertising to an LCL capabilities,
which is then used to resolve pause flow control settings.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>