xen/netback: remove not needed test in xenvif_tx_build_gops()

The tests for the number of grant mapping or copy operations reaching
the array size of the operations buffer at the end of the main loop in
xenvif_tx_build_gops() isn't needed.

The loop can handle at maximum MAX_PENDING_REQS transfer requests, as
XEN_RING_NR_UNCONSUMED_REQUESTS() is taking unsent responses into
consideration, too.

Remove the tests.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>