iwlwifi: mvm: bail out if CTDP start operation fails

We were assigning the return value of iwl_mvm_ctdp_command() to a
variable, but never checking it.  If this command fails, we should not
allow the interface up process to proceed, since it is potentially
dangerous to ignore thermal management requirements.

Fixes: commit 5c89e7bc

 ("iwlwifi: mvm: add registration to cooling device")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>