iwlwifi: speed up event log dumps

Take advantage of device's auto-increment for SRAM reads to eliminate extra
write address accesses.

Grab/release NIC access before/after entire read sequence, rather than for
each read individually.

After a quick check of dmesg logs, this seems to double Event Log dump speed,
reducing from about 20 milliseconds to about 10 milliseconds for 512 entries
using 3945.

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>