iwlwifi: return whether to stop from rfkill method

When indicating RF-kill toggle to the higher layer, that
may in turn call back to the transport (for MVM at least)
to turn off the device quickly. Instead of that, allow it
to return whether or not the device should be turned off,
this gets rid of the call indirection and will help make
the API more consistent when we go back to non-threaded
interrupts again for PCIe.

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