phy: mdio-thunder: Fix some Kconfig typos

Drop two extra occurrences of "on" in option title and help text.

Fixes: 379d7ac7

 ("phy: mdio-thunder: Add driver for Cavium Thunder SoC MDIO buses.")
Cc: David Daney <david.daney@cavium.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>