xen-netback: remove redundent parameter in netbk_count_requests

commit ac69c26e7accb04ae2cb9ab0872068983a42b3c8 upstream.

Tracking down from the caller, first_idx is always equal to vif->tx.req_cons.
Remove it to avoid confusion.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>