iwlwifi: simplify scheduler memory clearing

Writing 130 dwords into the device one by one is
rather inefficient, every one needs to lock, grab
NIC access (a few register reads/writes) and then
write the address and data registers.

Use the new memory clearing function to make this
easier and faster.

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