iwlwifi: move register access lock into transport

Move the reg_lock that protects HW register access
into the transport implementation. Locking is no
longer exposed, but handled internally in grab and
release NIC access. This simplifies the users.

Signed-off-by: Lilach Edelstein <lilach.edelstein@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>