xen-netback: respect user provided max_queues

xen-netback: respect user provided max_queues

Originally that parameter was always reset to num_online_cpus during
module initialisation, which renders it useless.

The fix is to only set max_queues to num_online_cpus when user has not
provided a value.

Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle>
Reported-by: Johnny Strom <johnny.strom@linuxsolutions.fi>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 4c82ac3c37363e8c4ded6a5fe1ec5fa756b34df3)
Signed-off-by: Annie Li <annie.li@oracle.com>