net: phy: add phy_interface_copy()

Add a helper for copying PHY interface bitmasks. This will be used by
the SFP bus code, which will then be moved to phylink in the subsequent
patches.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1uydVU-000000061W8-2IDT@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>