iwlagn: introduce silent grabbing of NIC access

There are a few cases like the WoWLAN support
I'm writing that require attempting to access
the NIC when it is known that it might not be
accessible, e.g. after the system woke up and
the platform might have reset the device.

To avoid messages in this case, introduce the
new function iwl_grab_nic_access_silent(), it
will only return an error status.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>