ixp4xx_eth: fix error check return value of platform_get_irq()

ixp4xx_eth: fix error check return value of platform_get_irq()

platform_get_irq() return negative value on failure, so null check of
return value is incorrect. Fix it by comparing whether it is less than
zero.

Fixes: 9055a2f59162 ("ixp4xx_eth: make ptp support a platform driver")
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220412085126.2532924-1-lv.ruyi@zte.com.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>