net: sun8i_emac: Fix overlong lines

When iterating over all RX/TX buffers, we were using a rather long "idx"
control variable, which lead to a nasty overlong line.

Replace "idx" with "i" to avoid this.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Amit Singh Tomar <amittomer25@gmail.com> # Pine64+
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>