phy: mdio-thunder: Fix some Kconfig typos

phy: mdio-thunder: Fix some Kconfig typos

BugLink: http://bugs.launchpad.net/bugs/1562968
Drop two extra occurrences of "on" in option title and help text.

Fixes: 379d7ac7ca31 ("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>
(cherry picked from commit e2ad1f976b721df383ff12c12a6dcc805cbb80f3)
Signed-off-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>