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: NAndreas Färber <afaerber@suse.de>
Acked-by: NDavid Daney <david.daney@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/phy/Kconfig

@@ -197,13 +197,13 @@ config MDIO_OCTEON
 	  drivers on some systems.
 
 config MDIO_THUNDER
-	tristate "Support for MDIO buses on on ThunderX SOCs"
+	tristate "Support for MDIO buses on ThunderX SOCs"
 	depends on 64BIT
 	depends on PCI
 	select MDIO_CAVIUM
 	help
 	  This driver supports the MDIO interfaces found on Cavium
-	  ThunderX SoCs when the MDIO bus device appears on as a PCI
+	  ThunderX SoCs when the MDIO bus device appears as a PCI
 	  device.