dpaa2-eth: Fix passing zero to 'PTR_ERR' warning

Fix smatch warning:

drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c:2419
 alloc_channel() warn: passing zero to 'ERR_PTR'

setup_dpcon() should return ERR_PTR(err) instead of zero in error
handling case.

Fixes: d7f5a9d8 ("dpaa2-eth: defer probe on object allocate")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>