rfkill: define rfill_soft_blocked() if !RFKILL

rfkill: define rfill_soft_blocked() if !RFKILL

If CONFIG_RFKILL is not set, the Intel WiFi driver will not build
the iw_mvm driver part due to the missing rfill_soft_blocked()
call. Adding a inline declaration of rfill_soft_blocked() if
CONFIG_RFKILL=n fixes the following error:

drivers/net/wireless/intel/iwlwifi/mvm/mvm.h: In function 'iwl_mvm_mei_set_sw_rfkill_state':
drivers/net/wireless/intel/iwlwifi/mvm/mvm.h:2215:38: error: implicit declaration of function 'rfkill_soft_blocked'; did you mean 'rfkill_blocked'? [-Werror=implicit-function-declaration]
 2215 |                 mvm->hw_registered ? rfkill_soft_blocked(mvm->hw->wiphy->rfkill) : false;
      |                                      ^~~~~~~~~~~~~~~~~~~
      |                                      rfkill_blocked

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reported-by: Neill Whillans <neill.whillans@codethink.co.uk>
Fixes: 5bc9a9dd7535 ("rfkill: allow to get the software rfkill state")
Link: https://lore.kernel.org/r/20220218093858.1245677-1-ben.dooks@codethink.co.uk
Signed-off-by: Johannes Berg <johannes.berg@intel.com>