net: macb: Fix passing zero to 'PTR_ERR'

Check PTR_ERR with IS_ERR to fix this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20201112144936.54776-1-yuehaibing@huawei.com


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