stmmac: remove dead option in the driver's Kconfig

This patch removes the CPU_SUBTYPE_ST40 dependency in the
driver's Kconfig.
In fact, this option has been removed in the commit:
 f9669187
as reported by Christian Dietrich.

Note that the driver remains tested on STM platforms, only.
Signed-off-by: NGiuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/stmmac/Kconfig

@@ -3,10 +3,10 @@ config STMMAC_ETH
 	select MII
 	select PHYLIB
 	select CRC32
-	depends on NETDEVICES && CPU_SUBTYPE_ST40
+	depends on NETDEVICES
 	help
 	  This is the driver for the Ethernet IPs are built around a
-	  Synopsys IP Core and fully tested on the STMicroelectronics
+	  Synopsys IP Core and only tested on the STMicroelectronics
 	  platforms.
 
 if STMMAC_ETH