iwlwifi: pcie: speed up the Tx DMA stop flow

We don't need to acquire MAC access for each access, it
makes much more sense to keep the MAC access. This speeds
up the Tx DMA stop flow significantly.
Moreover, if one channel can't be stopped, stop the others
but don't poll for them to avoid being stuck there for a
long time.

This solves a situation in which we were stuck in that flow
for way too long with a spinlock held which led to a kernel
panic.

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