nfp: Fix passing zero to 'PTR_ERR'

nfp_cpp_from_nfp6000_pcie() returns ERR_PTR() and never returns
NULL. The NULL test should be removed, also return correct err.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Link: https://lore.kernel.org/r/20201112145852.6580-1-yuehaibing@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>