iwlwifi: allow to zero SRAM

Sending a NULL pointer to iwl_trans_write_mem allows now
to zero SRAM.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc drivers/net/wireless/iwlwifi/iwl-trans.h
Simple merge

diff --cc drivers/net/wireless/iwlwifi/pcie/trans.c
Simple merge