iavf: Fix for setting queues to 0

Now setting combine to 0 will be rejected with the
appropriate error code.
This has been implemented by adding a condition that checks
the value of combine equal to zero.
Without this patch, when the user requested it, no error was
returned and combine was set to the default value for VF.

Fixes: 5520deb1

 ("iavf: Enable support for up to 16 queues")
Signed-off-by: Grzegorz Szczurek <grzegorzx.szczurek@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>