ravb: Fix int mask value overwritten issue

When RX/TX interrupt for Network Control queue and Best Effort queue
is issued at the same time, the interrupt mask of Network Control
queue will be reset when the mask of Best Effort queue is set.
This patch fixes this problem.

Signed-off-by: Masaru Nagai <masaru.nagai.vx@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>