Phonet: advise against enabling the pipe controller

As it currently is, the new code path is not compatible with existing
Nokia modems. This would break existing userspace for Nokia modem, such
as the existing oFono ISI driver.
Signed-off-by: NRémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/phonet/Kconfig

@@ -24,4 +24,5 @@ config PHONET_PIPECTRLR
 	  data with Nokia Slim modems like WG2.5 used on ST-Ericsson U8500
 	  platform.
 
-	  If unsure, say N.
+	  This option is incompatible with older Nokia modems.
+	  Say N here unless you really know what you are doing.