iwlegacy: always check if got h/w access before write

Before we write to the device register always check if
_il_grap_nic_access() was successful.

Change type return type _il_grap_nic_access() to bool, and
add likely()/unlikely() statements.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>