iwlwifi: pcie gen2: check iwl_pcie_gen2_set_tb() return value

If we use the iwl_pcie_gen2_set_tb() 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.

Fixes: ab6c6445 ("iwlwifi: pcie: copy TX functions to new transport")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>