iwlwifi: mvm: fix sparse warning when _DEBUGFS isn't set

Since the declaration of iwl_mvm_fw_error_rxf_dump and
iwl_mvm_fw_error_sram_dump is under
ifdef CONFIG_IWLWIFI_DEBUGFS, do the same for their
implementation.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>