net: phylink: fix PHY validation with rate adaption

Tim Harvey reports that link modes which he does not expect to be
supported are being advertised, and this is because of the workaround
we have for PHYs that switch interface modes.

Fix this up by checking whether rate matching will be used for the
requested interface mode, and if rate matching will be used, perform
validation only with the requested interface mode, rather than invoking
this workaround.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>