mac80211: call drv_stop only if driver is started

If drv_start() fails during hw_restart, all the running
interfaces are being closed/stopped, which results in
drv_stop() being called, although the driver was never
started successfully.

This might cause drivers to perform operations on uninitialized
memory (as they assume it was initialized on drv_start)

Consider the local->started flag, and call the driver's stop()
op only if drv_start() succeeded before.

Move drv_start() and drv_stop() to driver-ops.c, as they are no
longer simple wrappers.

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