Before we write to the device registers always check if
iwl_grap_nic_access() was successful.

On the way change return type of grab_nic_access() to bool, and add
likely()/unlikely() statement.
Signed-off-by: NStanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NWey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>