iwlwifi: missing error code in iwl_trans_pcie_alloc()

We don't set the error code here so we end up returning ERR_PTR(0) which
is NULL.  The caller doesn't expect that so it results in a NULL
dereference.

Fixes: 2e5d4a8f ("iwlwifi: pcie: Add new configuration to enable MSIX")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>