iwlwifi: print limited number of event log when uCode error

To help iwlagn uCode debugging, event log will dump to syslog when driver
detect uCode error occurred, but this only happen when compile with
CONFIG_IWLWIFI_DEBUG and debug flag is enabled; which is not always
the case. Also, there is another problem, if the flag is set, the entire
event log buffer will be dump to syslog, it can flood the syslog and
make it very difficult to debug the problem.

Change the default to only dump last 20 entries of event log to syslog
unless the following condition meets:
1. both compile with CONFIG_IWLWIFI_DEBUG and debug flag
is enabled, and then dump the entire event buffer to syslog.
2. dump event log request from debugfs

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Acked-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>