iwlwifi: mvm: fix wrong #ifdef that uses a macro from backports

I accidentally pushed a patch with CPTCFG (which is used in the
backports project) to the rs-fw.c file.  Fix that to use CONFIG
instead.

Fixes: 9f66a397

 ("iwlwifi: mvm: rs: add ops for the new rate scaling in the FW")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>