iwlwifi: mvm: move umac_error_event_table validity check to where it's set

There's no point in checking the validity of the
umac_error_event_table pointer every time we generate a dump.  It's
cleaner to do so when we read the value, namely when we receive the
alive data.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>