net: phylink: split phylink_sfp_module_insert()

Split out the configuration step from phylink_sfp_module_insert() so
we can re-use this later.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>