wifi: iwlwifi: skip affinity setting on non-SMP

Without SMP the function is just a stub that returns an error code. Add
a compile time check for CONFIG_SMP in the interest of not logging an
error if setting affinity is not possible anyway.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: Gregory Greenman <gregory.greenman@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240123200528.ed9094390731.Ic4e5e019c01fd4231b99cf4919af5d19d6353869@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>