iwlwifi: pcie: don't panic if pcie transport alloc fails

iwl_trans_pcie_alloc needs to return a non-zero value
if it fails. Otherwise the iwl_drv_start will think that
the allocation succeeded.
Remove the duplication of err and ret variable and use ret
which is the name we usually use in other places of the
driver.

Fixes: c278754a21e6 ("iwlwifi: mvm: support family 8000 B2/C steps")
CC: <stable@vger.kernel.org> [4.1]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>