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>