net: phy: Fix unsigned comparison with less than zero

Fix the following coccicheck warning:

./drivers/net/phy/at803x.c:493:5-10: WARNING: Unsigned expression
compared with zero: value < 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: 7beecaf7 ("net: phy: at803x: improve the WOL feature")
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/1635325191-101815-1-git-send-email-jiapeng.chong@linux.alibaba.com


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