net: dsa: b53: Add helper to set link parameters

Extract the logic from b53_adjust_link() responsible for overriding a
given port's link, speed, duplex and pause settings and make two helper
functions to set the port's configuration and the port's link settings.
We will make use of both, as separate functions while adding PHYLINK
support next.

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