drivers: net: xen-netfront: Simplify the calculation of variables

Fix the following coccicheck warnings:

./drivers/net/xen-netfront.c:1816:52-54: WARNING !A || A && B is
equivalent to !A || B.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/1612261069-13315-1-git-send-email-jiapeng.chong@linux.alibaba.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>