iwlwifi: pcie: fix mutex leak in gen2 start

If the context info fails to be allocated, the mutex
isn't unlocked properly, fix that.

Fixes: eda50cde

 ("iwlwifi: pcie: add context information support")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>