Make new L1CTL_FBSB_REQ work reliably

Make new L1CTL_FBSB_REQ work reliably

* port 'mobile' application to new l1ctl_tx_fbsb_req()
* make sure we have a proper downlinke header in front of l1ctl_fbsb_resp
* remove duplicate band_arfcn member of struct l1ctl_fbsb_resp
* reset the AFC to its default value when starting new FBSB task
* remove bogus l1s.sb.{synced.count} variables
* allocate msg and send l1ctl_fbsb_resp() only from process context, not FIQ
* properly report SNR and BSIC in fbsb_resp
* introduce arbitrary SNR thresholds for FB0->FB1 and FB1->SB switching