octeontx2-af: cn10k: Do not enable RPM loopback for LPC interfaces

Internal looback is not supported to low rate LPCS interface like
SGMII/QSGMII. Hence don't allow to enable for such interfaces.

Fixes: 3ad3f8f9 ("octeontx2-af: cn10k: MAC internal loopback support")
Signed-off-by: Geetha sowjanya <gakula@marvell.com>
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>