iwlwifi: Limit size of Event Log dump

If device provides bad values for Event Log parameters (due to being asleep
or SRAM corruption, etc.), the size can be very, very large (e.g. 0xa5a5a5a5),
which can flood system log.

Sanity-check capacity and next_entry values and limit to reasonable size dump.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>