iwlwifi: pcie: lock start_hw / start_fw / stop_device

This allows to ensure that we don't have races between them.
A user reported that stop_device was called twice upon
rfkill interrupt after suspend. When the interrupts are
enabled, and right after when we directly check the rfkill
state.

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