iwlwifi: mvm: get rid of tx_path_lock

TX path lock was introduced in order to prevent out of order
invocations of TX.

This can happen in the following flow:

TX path invoked from net dev
Packet dequeued
	TX path invoked from RX path
	Packet dequeued
	Packet TXed
Packet TXed

However, we don't really need a lock. If TX path is already
invoked from some location, other paths can simply abort their
execution, instead of waiting to the first path to finish, and
then discover queue is (likely) empty or stopped.

Replace the lock with an atomic variable to track TX ownership.
This simplifies the locking dependencies between RX and TX paths,
and should improve performance.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>