iwlwifi: mvm: fix compilation with IWLWIFI_DEBUGFS not set

The commits below broke compilation when
CONFIG_IWLWIFI_DEBUGFS is not set.
FIx that.

Fixes: ddf89ab1 ("iwlwifi: mvm: allow to force the Rx chains from debugfs")
Fixes: 9d761fd8

 ("iwlwifi: mvm: add trigger for firmware dump upon missed beacons")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>