net: lan966x: fix checking for return value of platform_get_irq_byname()

The platform_get_irq_byname() returns non-zero IRQ number
or negative error number. "if (irq)" always true, chang it
to "if (irq > 0)"

Signed-off-by: Li Qiong <liqiong@nfschina.com>
Signed-off-by: David S. Miller <davem@davemloft.net>