net: bql: allow the config to be disabled

It is impossible to disable BQL individually today, since there is no
prompt for the Kconfig entry, so, the BQL is always enabled if SYSFS is
enabled.

Create a prompt entry for BQL, so, it could be enabled or disabled at
build time independently of SYSFS.
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/Kconfig

@@ -331,6 +331,7 @@ config NET_RX_BUSY_POLL
 
 config BQL
 	bool
+	prompt "Enable Byte Queue Limits"
 	depends on SYSFS
 	select DQL
 	default y