iwlwifi: pcie: check iwl_pcie_txq_build_tfd() return value

If we use the iwl_pcie_txq_build_tfd() return value for BIT(),
we should validate that it's not going to be negative, so do
the check and bail out if we hit an error. We shouldn't, as
we check if it'll fit beforehand, but better be safe.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>